The load in the electrical circuit is characterized by the current power, the current measurement in amperes. The current strength sometimes has to be measured to test the permissible magnitude of the load on the cable. For laying the electrical line, cables of different sections are used. If the cable works with a load above the permissible value, then it is heated, and the insulation is gradually collapsed. As a result, this leads to and replacing the cable.

  • After laying a new cable, you need to measure the current passing through it with all running electrical devices.
  • If an additional load is connected to the old wiring, then you should also check the current value that should not exceed the permissible limits.
  • With the load equal to the upper permissible limit, the current correspondence occurs through. Its value should not exceed the nominal value of the operating current of the machine guns. Otherwise, the circuit breaker will de-energize the network due to overload.
  • The current measurement is also necessary to determine the operation modes of electrical devices. Measuring the current load of electric motors is performed not only to check their performance, but also to detect the load above the permissible, which may occur due to the large mechanical effort when the device is working.
  • If you measure the current in the chain of the working, then it will show the serviceability.
  • The performance in the apartment is also checked by the current measurement.
Current power

In addition to the current, there is the concept of current power. This parameter determines the current operation per unit time. The power of the current is equal to the ratio of the work performed by the time interval for which this work was performed. Denote by the letter "P" and measured in watts.

Power is calculated by multiplying the voltage of the network for the current consumed by the connected electrical devices: P \u003d U X I. Typically, the electrical devices indicate the power consumed by which the current can be defined. If your TV has a power of 140 W, then to determine the current divide this value by 220 V, as a result we get 0.64 amps. This is the value of the maximum current, in practice the current may be less when a decrease in the brightness of the screen or other changes in settings.

Measuring current to devices

To determine the consumption of electrical energy, taking into account the operation of consumers in different modes, electrical measuring instruments are needed, capable of measuring the current parameters.

  • . To measure the values \u200b\u200bof the current in the chain, special devices are used, called ammeters. They are included in the measured chain on a serial scheme. The internal resistance of the ammeter is very small, so it does not affect the parameters of the chain. The ammeter can be marked in amperes or other shares of the amperes: microampers, milliamperes, etc. There are several types of ammeters: electronic, mechanical, etc.
  • It is an electronic measuring device capable of measuring different parameters of the electrical circuit (resistance, voltage, conductor break, battery suitable, etc.), including current strength. There are two types of multimeters: digital and analog. In the multimeter there are various measurement settings.

The procedure for measuring current force by a multimeter:
  • Find out what the measurement interval of your multimeter. Each device is designed to measure the current in some interval, which must correspond to the measured electrical circuit. The greatest allowable measurement current must be specified in the instructions.
  • Select the appropriate measurement mode. Many multimeters are capable of working in different modes, and measure different values. For measurements, current strength must be switched to the appropriate mode, considering the current (permanent or variable).
  • Install the necessary measurement interval on the device. It is better to establish the upper limit of the current strength somewhat above the alleged value. You can reduce this limit at any time. But it will be a guarantee that you will not bring the device in order.
  • Insert the measuring plugs of wires into the socket. The device includes two wires with affairs and connectors. Nests must be marked on the instrument or depicted in the passport.

  • To start the measurement, you must connect the multimeter to the chain. This should follow the safety rules and do not touch the current parts of the unprotected parts of the body. It is impossible to measure in a humid environment, since the moisture conducts an electric current. On the hands you should wear rubber gloves. To break the chain for measuring, you should cut the conductor and deteriorate insulation at both ends. Then connect the multimeter probe to the stripped ends of the wire and make sure in good contact.
  • Turn on the circuit power and fix the instrument readings. If necessary, adjust the upper measurement limit.
  • Disable the power circuit and disconnect the multimeter.
  • . If you need to measure the current without breaking the electrical circuit, the measuring ticks will be an excellent option for performing this task. This device produces several species, and different designs. Some models can measure other parameters of the chain. Use the measuring current ticks is very convenient.

Ways to measure current

To measure the current force in the electrical circuit, one ampermeter is necessary or another instrument that can measure the current strength, connect to the positive terminal of the current source or, and the other output to the consumer's wire. After that, the current strength can be measured.

When measuring, it is necessary to observe accuracy, since an electric arc may occur when the operating electrical circuit is broken.

To measure the power of the electrical devices connected directly to the outlet or the household network cable, the measuring device is configured to an AC mode with an overpriced upper boundary. Then the measuring instrument is connected to the gap of the phase wire.

All connections and disconnection work is allowed only in a de-energized chain. After all connections, you can feed meals and measure the current strength. At the same time it is impossible to concern the bare-cutting parts, in order to avoid electric shock. Such measurement methods are inconvenient and create a certain danger.

It is much more convenient to measure the measurements by current-measuring tongs that can perform all the functions of the multimeter, depending on the execution of the device. It is very simple to work with such ticks. You need to configure the measurement mode of a direct or alternating current, dilute the mustache and cover the phase wire. Then you need to check the density of the ordinance of the mustache between themselves and measure the current. For correct readings, it is necessary to cover only the phase wire. If you cover two wires at once, then measurements will not work.

Current testers serve only for measurement of alternating current parameters. If they are used to measure direct current, then the mustache will be freezed with a lot of force, and it will be possible to push them only by turning off the power.

Under the electrical voltage, the work performed by an electric field to move the charge with a strength of 1 CL (pendant) from one point of the conductor to another.

How does the voltage occur?

All substances consist of atoms, which are a positively charged core, around which smaller negative electrons are circling at high speed. In the general case, the atoms are neutral, since the number of electrons coincides with the number of protons in the nucleus.

However, if some electrons take away from atoms, they will strive to attract the same amount by forming a plus field around them. If adding electrons, then their excess will arise, and the negative field. Potentials are formed - positive and negative.

With their interaction, mutual attraction will arise.

The greater the amount of the difference - the potential difference is the stronger the electrons from the material with their redundant content will be dragging to the material with their disadvantage. The stronger the electric field and its voltage will be.

If you connect potentials with different charges of conductors, then electric - directional motion of charge carriers, seeking to eliminate the difference in potentials. To move along conduit charges, the power of the electric field is carried out, which is characterized by the concept of electrical voltage.

What is measured

Temperatures;

Types of voltage

Constant pressure

The voltage in the electrical network is constantly when it is always positive potential from one side, and on the other - negative. Electric in this case has one direction and is constant.

The voltage in the DC circuit is defined as the potential difference at its ends.

When the load is connected to the DC circuit, it is important not to confuse the contacts, otherwise the device may fail. A classic example of a constant voltage source is batteries. Networks are used when it is not necessary to transmit energy over long distances: in all types of transport - from motorcycles to spacecraft, in military equipment, electric power industry and telecommunications, in emergency electricity, in industry (electrolysis, smell in arc electric housing, etc.) .

AC voltage

If periodically change the polarity of potentials, or move them in space, then the electric will rush in the opposite direction. The number of such changes in the direction during a certain time shows the characteristic called frequency. For example, standard 50 mean that the polarity of the voltage in the network changes in a second 50 times.


The voltage in the electrical networks of AC is a time function.

Most often used the law of sinusoidal oscillations.

This is obtained due to the fact that it occurs in the coil of asynchronous engines due to the rotation of the electromagnet around it. If you expand the rotation of time, then the sinusoid is obtained.

It consists of four wires - three phase and one zero. The voltage between the wires zero and phase is 220 V and called phase. Between phase, the voltage also exists, is called linear and equal to 380 V (the potential difference between two phase wires). Depending on the type of connection in a three-phase network, you can get a phase voltage or linear.

Power measurement. In DC circuits, power is measured with an electric or ferrodynamic wattmeter. Power can also be calculated by multiplying current and voltage values \u200b\u200bmeasured by an ammeter and voltmeter.

In single-phase current circuits, power measurement can be carried out by an electrodynamic, ferrodynamic or induction wattmeter. Wattmeter 4 (Fig. 336) has two coils: current 2, which turns on in the circuit sequentially, and voltages 3, which is turned on in the chain in parallel.

The wattmeter is an instrument requiring the inclusion of the correct polarity, so its generator clamps (clamps to which the conductors connect from the source 1) are indicated by asterisks.

Fig. 336. Power Measurement Scheme

To expand the limits of measurement of wattmeters, their current coils are included in the circuit using shunts or measuring current transformers, and voltage coils through additional resistors or stress transformers.

Measurement of electrical energy. Measurement method. To account for electrical energy obtained by consumers or current sources, electric power meters are used. Electrical energy meter on the principle of its action is similar to the wattmeter. However, unlike the wattmeters, instead of a spiral spring, creating a opposing moment, in the meters involve a device similar to an electromagnetic damper, creating a braking force proportional to the frequency of rotation of the mobile system. Therefore, when the device is turned on in an electrical circuit, the resulting torque will cause no deviation of the movable system to some angle, but its rotation with a certain frequency.

The number of revolutions of the movable part of the device will be proportional to the product of the power of the electric current at the time during which it acts, i.e. the number of electrical energy passing through the device. The number of counter revolutions is fixed by the counting mechanism. The gear ratio of this mechanism is chosen so that the meter readings can be counted no turnover, but directly electrical energy in kilowatt-hours.

Ferrodynamic and induction counters received the greatest distribution; The first is used in DC circuits, the second - in alternating current circuits. Electrical energy meters include direct and alternating circuits in the electrical circuits as well as wattmeters.

Ferrodynamic counter (Fig. 337) are installed on er p. p. direct current. It has two coils: motionless 4 and mobile 6. The fixed current coil 4 is divided into two parts, which cover the ferromagnetic core 5 (usually from Permalloe). The latter allows you to create a strong magnetic field in the device and a significant torque that ensures the normal operation of the meter in the conditions of shaking and vibrations. The use of Permalloe helps to reduce the error of the counting mechanism 2 from the hysteresis of the magnetic system (it has a very narrow hysteresis loop).


To reduce the effect of external magnetic fields on the meter readings, the magnetic streams of individual parts of the current coil have a mutually opposite direction (astatic system). In this case, the external field, weakening the stream of one part, respectively, enhances the stream of the other part and overall a small effect on the resulting torque generated by the device. The movable coil 6 of the counter (voltage coil) is anchored, made in the form of a disk from an insulating material or as an aluminum bowl. The coil consists of separate sections connected to the collector plates 7 (these compounds in Fig. 337 are not shown), according to which brushes from thin silver plates slide.

The ferrodynamic counter works in principle as a DC motor, the winding of the anchor of which is connected in parallel, and the excitation winding is sequentially with the electricity consumer. Anchor rotates in the air gap between the poles of the core. The braking torque is created as a result of the interaction of the flow of a constant magnet 1 with vortex currents arising in an aluminum disk 3 during its rotation.

To compensate for the effect of friction and decreases, due to this, the error of the device in ferrodynamic counters is set to a compensation coil or a magnetic field of a fixed (current) coil placed petal from permallium, which has a high magnetic permeability at low field strength. With low loads, this petal enhances the magnetic current of the current coil, which leads to an increase in the torque and friction compensation. With an increase in the load, the induction of the magnetic field of the coil increases, the petal is saturated and its compensating effect ceases to increase.

When working counter on er p. p. Strong shocks and shocks are possible, at which brushes can bounce from collector plates. In this case, the brushes will be sparking. To prevent it between brush, includes a condenser with and resistor R1. Compensation of the temperature error is carried out using the RT thermistor (semiconductor device, the resistance of which depends on temperature). It is included in conjunction with the addition resistor R2 parallel to the movable coil. To reduce the effect of shaking and vibrations to work counters, they are installed on er. p. p. on rubberometallic shock absorbers.

Induction counter It has two electromagnets (Fig. 338, a), between which the aluminum disk is located 7. The torque in the device is created as a result of the interaction of variables of magnetic fluxes F1 and F2, created by electromagnet coils, with vortex currents I B1 and I B2 induced by them in aluminum disk (as well as in the usual induction measuring mechanism, see § 99).

In the induction meter, the torque M must be proportional to the power P \u003d uicos?. For this, the coil 6 of one of the electromagnets (current) is included in series with a load of 5, and the coil is 2 other (voltage coil) - parallel to the load. In this case, the magnetic flow F1 will be proportional to the current I in the load circuit, and the F2-voltage stream is applied to the load. To ensure the required phase shift angle? between F1 and F2 streams (so that sin? \u003d cos?) In the electromagnation of the voltage coils, the magnetic shunt 3 is provided, through which part of the F2 flow is closed

Fig. 337. Ferrodynamic Electric Energy Counter

Fig. 338. Induction Electric Energy Counter

in addition to the disk 7. The phase shift angle between F1 and F2 flows is precisely regulated by changing the position of the metal screen 1 located on the flow path that is branched through the magnetic shunt 3.

The braking moment is created in the same way as in the Ferrodynamic meter. Compensation of the torque moment is carried out by creating a small asymmetry in a magnetic chain of one of the electromagnets using a steel screw.

To prevent the anchor rotation in the absence of a load under the action of the force created by the device compensating for friction, the steel brake hook is strengthened on the axis of the meter. This hook is attracted to the brake magnet 4, due to which the possibility of rotating the movable system without load is prevented.

When the meter is running under the load, the brake hook practically does not affect its testimony.

To rotate the counter in the desired direction, it is necessary to comply with a certain order of connecting the wires to its clips. The load clips of the device to which the wires running from the consumer are denoted by letters I (Fig. 338, b), the generator clamps to which the wires from the current source or from the AC network are connected - letters G.

Current measurement.To measure the current in the Ampmeter 2 chain (Fig. 332, a) or milliammer meter in the electrical circuit, with a receiver 3 of electrical energy.

In order for the inclusion of an ammeter, there is no effect on the operation of electrical installations and it did not create large losses of energy, ammeters are performed with small internal resistance. Therefore, almost resistance can be considered equal to zero and neglect the voltage caused by it. The ammeter can be turned on in the chain only sequentially with the load. If the ammeter is connected directly to the source 1, then through the coil of the device will go very high current (the ammeter resistance is not enough) and it burns.

To expand the limits of measurement of ammeters intended for operation in the DC circuits, they are included in the chain parallel to the shunt 4 (Fig. 332, b). In this case, only part I and the measured current I are passed through the device, inversely proportional to its resistance R A. B aboutthe extended part I w shutters through the shunt. The device measures the voltage drop on the shunt, depending on the current passing through the shunt, i.e. it is used as a malelololtmeter. The scale of the device is graded in amperes. Knowing the resistance of the device R a and the shunt R w can be in the current I A A fixed by the device, to determine the measured current:

I \u003d i a (r a + r w) / r sh \u003d i and n (105)

where n \u003d i / i a \u003d (r a + r sh) / r w is the coefficient of shunting. It is usually chosen equal to or multiple 10. The spun resistance required for measuring the current I, in N times greater than the current of the device I,

R Ш \u003d R \u200b\u200ba / (n-1) (106)

Constructively shunts are either mounted in the instrument housing (shunts per currents up to 50 A), or installed outside it and connected to the device with wires. If the device is designed for permanent operation with a shunt, then its scale is graded immediately in the values \u200b\u200bof the current current, taking into account the shunt coefficient and no calculations for determining the current is not required. In the case of applying external (individual devices), the shunts indicate the rated current to which they are calculated, and the rated voltage on the clips (calibrated shunts). According to the standards, this voltage can be 45, 75, 100 and 150 mV. Shunts are selected to the instruments so that at rated voltages on the shunt clips, the arrow of the device deviated to the entire scale. Consequently, the nominal stresses of the device and the shunt should be the same. There are also individual shunts designed to work with a specific device. Shunts are divided into five accuracy classes (0.02; 0.05; 0.1; 0.2; 0.5). Class designation meets percentage permissible error.

In order to increase the cowage temperature during current passage, there was no effect on the testimony of the instrument, the shunts are made of materials with high resistivity and a small temperature coefficient (Konstanta, Manganin, Nickelin, etc.). To reduce the effect of temperature on the ammeter readings, in sequentially with the coil of the device, in some cases, an additional resistor from constant-tana or other similar material is included.

Fig. 332. Schemes for measuring the current (A, B) and voltage (B, D)

Voltage measurement. To measure the voltage U, acting between any two points of the electrical circuit, Voltmeter 2 (Fig. 332, B) attach to these points, i.e., parallel to the source 1 of electrical energy or receiver 3.

In order for the inclusion of the voltmeter, there was no effect on the operation of electrical installations and it did not create large losses of energy, voltmeters are performed with greater resistance. Therefore, it is practically possible to neglect the current passing on the voltmeter.

To expand the limits of the voltmeter measurement, the addition resistor 4 (R D) is sequentially with the winding of the device (R / Fig. 332, d). At the same time, only a part of the U V of the measured voltage U is accounting for the device, proportional to the resistance of the R V instrument.

Knowing the resistance of the added resistor and the voltmeter, by the value of the voltage U V fixed by the voltmeter, determine the voltage acting in the chain:

U \u003d (R V + R. D. ) / R. V. * U. V. \u003d Nu. V. (107)

The value of N \u003d U / U V \u003d (R V + R D) / R V shows how many times the measured voltage U is more voltage U v, which occurs on the device, i.e., how many times the limit of the voltage measurement limit is increasing when an additional resistor.

The resistance of the added resistor necessary to measure the voltage U, in the variety of the larger voltage of the UV instrument, is determined by the formula R d \u003d (n-1) R v.

The added resistor can be integrated into the instrument and simultaneously used to reduce the effect of the ambient temperature on the instrument readings. For this purpose, the resistor is made of a material having a small temperature coefficient, and its resistance significantly exceeds the coil resistance, as a result of which the overall resistance of the device becomes almost independent of temperature change. According to accuracy, additional resistors are divided into the same accuracy classes as shunts.

Voltage divisors.To expand voltmeter measurement limits, voltage dividers are also used. They allow you to reduce the voltage to be measured to a value corresponding to the nominal voltage voltmeter (limit voltage on its scale). The ratio of the input voltage of the divider U 1 to the output U 2 (Fig. 333, a) is called fission coefficient. At idling U 1 / U 2 \u003d (R 1 + R 2) / R2 \u003d 1 + R 1 / R 2. In voltage divisors, this ratio can be selected equal to 10, 100, 500, etc., depending on which

Fig. 333. Voltage divider inclusion schemes

the outputs of the divider is connected to the voltmeter (Fig. 333, b). The voltage divider makes a small error in the measurement only if the resistance of the voltmeter R v is sufficiently large (current passing through the divider, small), and the resistance of the source to which the divider is connected, is not enough.

Measuring transformers.To enable electrical measuring devices in the AC circuit, measuring transformers are used to ensure the safety of the service personnel when performing electrical measurements in high voltage circuits. The inclusion of electrical measuring devices into these chains without such transformers is prohibited by safety regulations. In addition, measuring transformers expand the limits of measuring instruments, i.e., it allows you to measure large currents and voltages using simple devices calculated to measure small currents and stresses.

Measuring transformers are divided into voltage transformers and current transformers. Voltage transformer 1 (Fig. 334, a) is used to connect voltmeters and other devices that should react to voltage. It is performed as the usual two-winding lowering transformer: the primary winding is connected to two points, between which the voltage is required, and the secondary to the voltmeter 2.

In the diagrams, the voltage transformer is depicted as an ordinary transformer (in Fig. 334, and is shown in the circle).

Since the resistance of the voltmeter winding connected to the voltage transformer is large, the transformer works practically in idling mode, and can be considered with a sufficient degree of accuracy that the voltages U 1 and U 2 on the primary and secondary windings will be directly proportional to the number of turns? 1 and? 2 of both transformer windings, i.e.

U 1 / u 2 \u003d? one /? 2 \u003d N. (108)

Thus, pick up the appropriate number of turns? 1 and? 2 transformer windings, high voltages can be measured, submitting small voltages to the electrical measuring device.

The U 1 voltage can be determined by multiplying the measured secondary voltage U 2 to the transformation coefficient of the transformer N.

Voltmeters intended for permanent operation with voltage transformers are graded at the factory based on the transformation coefficient, and the values \u200b\u200bof the measured voltage can be directly counted on the instrument scale.

To prevent the danger of damage to the service personnel by electric shock in case of damage to the transformer isolation, one element of its secondary winding and steel transformer casing must be grounded.

Current transformer 3 (Fig. 334, b) serves to connect ammeters and other devices that should react to the variable current flowing through the chain. It is performed in the form

Fig. 334. Inclusion of electrical instruments by means of measuring voltage transformers (A) and current (b)

conventional two-winding increases transformer; The primary winding is turned on in series in the measured current circuit, an ammeter 4 is connected to the secondary winding.

The circuit designation of the measuring transformers of the current is shown in Fig. 334, B in a circle.

Since the resistance of the ammeter winding connected to the current transformer is usually small, the transformer practically operates in a short circuit mode, and with a sufficient degree of accuracy, we can assume that currents I 1 and I 2 passing along its windings will be inversely proportional to the number of turns? 1 and? 2 of these windings, i.e.

I 1 / i 2 \u003d? one /? 2 \u003d N. (109)

Consequently, choose the number of turns appropriately? 1 and? 2 transformer windings, you can measure large currents I 1, passing through an electrical measuring device Small currents I 2. Current I 1 may be determined by multiplying the measured secondary current I 2 by n.

Ampmeters intended for permanent operation together with current transformers are graded at the factory based on the transformation coefficient, and the values \u200b\u200bof the measured current i 1 can be directly counted on the instrument scale.

To prevent the danger of damage to the service personnel in the case of damage to the insulation of the transformer, one of the clips of the secondary winding and the transformer casing grounds.

On er p. p. Using the so-called current pass transformers (Fig. 335). In such a transformer, the magnetic pipe 3 and the secondary winding 2 are mounted on the passage insulator 4, which is serviced to input high voltage in the body, and the role of the primary winding of the transformer performs a copper rod 1 passing inside the insulator.

Conditions for current transformers are different from ordinary. For example, the opening of the secondary winding of the current transformer when the primary winding is turned on is unacceptable, as this will cause a significant increase in the magnetic flux and, as a result, the temperature of the core and the transformer winding, that is, the output of it. In addition, a large event may be induced in an open secondary winding of the transformer. C, dangerous for the personnel of measurement.

When the instruments are turned on by means of measuring transformers, there are errors of two types: error in the transformation ratio and the angular error (with a change in voltage or current of the ratio 1 / U 2 and I 1 / i 2, the phase shift angle between primary and secondary voltages and currents deviates 180 °). These errors increase with the load of the transformer above the nominal. The angular error affects the results of the measurement

Fig. 335. Tower Measuring current transformer

relations of which depend on the phase shift angle between voltage and current (for example, wattmeters, electric energy meters, etc.). Depending on the errors allowed, the measuring transformers are divided into accuracy classes. Accuracy class (0.2; 0.5; 1, etc.) corresponds to the greatest permanent error in the transformation ratio as a percentage of its nominal value.

Measurement by the method of ammeter and voltmeter. The resistance of any electrical installation or section of the electrical circuit can be determined using an ammeter and a voltmeter using the Ohm's law. When the instruments are turned on according to the fig. 339, and via an ammeter passes not only the measured current I x, but also the current I V flowing through the voltmeter. Therefore resistance

R x \u003d u / (i - u / r v ) (110)

where R V. - Voltmeter resistance.

When the instruments are turned on according to the fig. 339, B Voltmeter will measure not only the drop in voltage UX on a certain resistance, but also a voltage drop in the ammeter winding U a \u003d IR A. So

R x \u003d u / i - r a (111)

where R A. - Ammeter resistance.

In cases where the resistances of the devices are unknown and, therefore, cannot be taken into account, it is necessary when measuring low resistances to use the circuit in Fig. 339, and, and when measuring large resistance - a diagram of Fig. 339, b. In this case, the measurement error defined in the first diagram of the current I V, and in the second - voltage drop of the UA, will be small compared to the current i x and the voltage U x.

Measurement of resistance by electric bridges. The bridge circuit (Fig. 340, a) consists of a power source, a sensitive instrument (galvanometer d) and four resistors included in the bridge shoulders: with an unknown resistance R x (R4) and known resistances R1, R2, R3, which may during measurements change. The device is included in one of the diagonals of the bridge (measuring), and the power source to another (nourishing).

Resistance R1 R2 and R3 can be chosen such that when contacting the contact in the instrument readings will be zero (in

Fig. 339. Schemes for measuring the resistance by the method of ammeter and voltmeter

Fig. 340. DC bridge circuits used to measure resistance

commission is customary to say that the bridge is balanced). At the same time unknown resistance

R x \u003d (R 1 / R 2) R 3 (112)

In some bridges, the ratio of the shoulders R1 / R2 is installed constant, and the balance of the bridge is achieved only by the selection of resistance R3. In others, on the contrary, the resistance R3 is constantly, and the equilibrium is achieved by the selection of resistance R1 and R2.

Measuring resistance to the DC bridge is as follows. Unknown resistance R x attach to clamps 1 and 2 (for example, the winding of the electric machine or the machine), to the clamps 3 and 4 - the galvanometer, and the power supply (dry galvanic element or battery). Then, changing the resistance R1, R2 and R3 (which uses resistance stores that are switched by the corresponding contacts), the bridge equilibrium is achieved, which is determined by zero indication of the galvanometer (with a closed contact B).

There are various designs of DC bridges, when using which no calculations are required, since the unknown resistance R x is counted on the instrument scale. Resistance stores mounted in them allow measuring resistance from 10 to 100,000 Ohm.

When measuring the low resistances, the conventional bridges of the resistance of the connecting wires and contact compounds make large errors in the measurement results. For their elimination, double DC bridges are used (Fig. 340, b). In these bridges, the wire connecting the resistor with the measured resistance R x and some sample resistor with R0 resistance with other bridge resistors, and their contact compounds are included in series with resistors of the appropriate shoulders, the resistance of which is installed at least 10 ohms. Therefore, they practically do not affect the measurement results. The wires connecting resistors with resistances R x and R0 are included in the power circuit and do not affect the equilibrium conditions of the bridge. Therefore, the accuracy of measurement of small resistance is quite high. The bridge is performed so that when adjusted it is respected by the following conditions: R1 \u003d R2 and R3 \u003d R4. In this case

R x \u003d R 0 R 1 / R 4 (113)

Double bridges allow for measuring resistance from 10 to 0.000001 Ohm.

If the bridge is not balanced, then the arrow in the galvanometer will deviate from the zero position, since the current of the measuring diagonal at constant values \u200b\u200bof the resistance R1, R2, R3 and E. d. s. The current source will depend on the resistance of R x. This allows you to chant the galvanometer scale in the resistance units R x or any other units (temperature, pressure, etc.), on which it depends on the resistance. Therefore, the unbalanced DC bridge is widely used in various devices for measuring non-electrical magnitudes by electrical methods.

Also different AC bridges are also used, which make it possible to measure with a large accuracy of inductance and containers.

Measurement by an omeme. Ommeter represents a milliampermeter 1 with a magnetoelectric measuring mechanism and is turned on in series with the measured resistance R x (Fig. 341) and an additional resistor R D into a DC chain. With unchanged er d. s. The source and resistance of the resistor R D current in the chain depends only on the resistance R x. This allows you to reward the device to the scale directly in Oma. If the output clamps of the device 2 and 3 are closed in short (see bar line), then the current I in the maximum chain and the device arrow deviates to the right to the highest angle; On the scale it corresponds to the resistance equal to zero. If the instrument circuit is open, then i \u003d 0 and the arrow is at the beginning of the scale; This provision corresponds to resistance equal to infinity.

The device is powered by a dry galvanic element 4, which is installed in the instrument housing. The device will give correct indications only if the current source has unchanged e. d. s. (the same as during the graduation of the scale of the device). Some ommers have two or more measurement limits, for example from 0 to 100 ohms and from 0 to 10,000 Ohm. Depending on this, the resistor with the measured resistance R x is connected to various clamps.

Mega-resistant measurement by megohmmeters.Megaommeters of the magnetoelectric system are most often used to measure insulation resistance. As the measuring mechanism, the logometer 2 is used in them (Fig. 342), which

Fig. 341. Oncemmeter Inclusion Scheme

Fig. 342. Megaommeter device

ryo does not depend on the voltage of the current source that feeds the measuring chains. Coils 1 and 3 devices are in a magnetic field of a permanent magnet and connected to a common power source 4.

Sequentially with one coil includes an additional resistor R D, into a chain of another coil - resistor resistance R x.

As a source, a small direct current generator 4 is usually used, called inducer; The anchor of the generator leads to rotation by the handle connected to it through the gearbox. Inductors have significant stresses from 250 to 2500 V, due to which the megoomemeter can be measured by large resistance.

When the interaction of Currents I1 and I2 occur with the magnetic field of a permanent magnet, two opposite directed moments M1 and M2 are created, under the influence of which the movable part of the device and the arrow will occupy a certain position. As shown in § 100, the position of mobile

Fig. 343. General view of the megohometer (a) and its simplified scheme (b)

parts of the logometer depends on the ratio I1 / I2. Consequently, when changing R x will the angle will change? Deviations arrows. The megaommeter scale is graded directly in kiloma or megaoms (Fig. 343, a).

To measure the insulation resistance between the wires, you must turn them off from the current source (from the network) and attach one wire to the clamp L (line) (Fig. 343, b), and the other - to the clamp 3 (ground). Then, rotating the inductor handle 1 of the megaommeter, is determined on the scale of the Logometer 2 insulation resistance. Instrument switch 3 allows you to change the measurement limits. Inductor voltage, and therefore, the frequency of rotation of its handle theoretically does not affect the measurement results, but it is practically recommended to rotate it more or less evenly.

When measuring the insulation resistance between the windings of the electrical machine, they are disconnected from each other and combine one of them with a clip of L, and the other with a clamp 3, after which rotating the inductor handle, determine the insulation resistance. When measuring the insulation resistance of the winding relative to the body, it is connected to the clamp 3, and the winding - with the clip of L.

Charged particles, getting into the electric field, begin to move ordered in a specific direction. Particles acquire a certain energy, that is, work is performed. To determine the magnitude of the work on the movement of electrical charges in the electric field with tension E. It took the introduction of another physical size - electrical voltage U..

What is the operation of the electric field

Attitude of work BUTcommitted by any electric field when moving a positive charge from one point of the field to another, to the size of the charge q. called electrical stress U. between these points:

$$ U \u003d (A \\ OVER Q) $$

It can be said that the electrical stress is equal to the work on the movement of the charge in 1 pendant from one point of the electric field to another.

Then to determine the magnitude of the perfect field of work, you can get the following expression:

$$ a \u003d (Q * U) $$

Fig. 1. Electrons in the electric field.

Units

In the international system of units (SI), the voltage measurement unit (B) is named after the Italian researcher Alessandro Volta (1745-1827), which made a huge contribution to the understanding of the nature of electricity. Since work is measured in Joules (J), and the charge in the coulon (K), then:

$$ \u003d (\\ Over) $$

The voltage may vary in the widest limits, therefore such non-system units are often used for calculations as:

  • 1 microvolt (MKV) \u003d 0.0000001 V;
  • 1 Millivolt (MV) \u003d 0.001 V;
  • 1 kilovolt (kV) \u003d 1000 V;
  • 1 mV (megavolt) \u003d 1000000 V.

Constant and alternating voltage

There are two types of stresses - constant and variable. An example of the sources of constant voltage can be the usual batteries used in household appliances: consoles, phones, etc. On the surface of the batteries there are always designations "-" and "+".

This means that the direction of the electric field created by the battery will be constant all the time. Sources of alternating voltage were invented later and got a huge distribution due to the fact that alternating current is easier to transform (strengthening, weakening) and transmitted to long distances.

Fig. 2. Charts of constant and alternating voltages.

From the graphs it is clear that constant voltage does not depend on the time

$$ U (T) \u003d const $$

A variable voltage changes, turning through the zero value by changing the sign "+" to "-". For the electrical voltage formula u (t), trigonometric functions of sinus or cosine are well suited:

$$ U (t) \u003d u_a * sin (ω * t) $$

where u a — amplitude of alternating voltage, i.e. the maximum voltage value;

ω - The frequency of alternating voltage, showing how many times in one second changes the voltage sign, that is, "plus" changes to "minus". The value of the frequency shows at what speed (as often) the polarity of the voltage varies. For example, in the electrical outlets of our apartments, the voltage varies 50 times per second (with a frequency of 50 hertz).

The effect of electrical voltage, starting with some values \u200b\u200bbecomes unsafe for humans. In dry rooms, safe is considered to be voltage up to 36 V. For premises with elevated dampness, this value is even less - 12 V. Therefore, it is necessary to always follow safety techniques when working and handling electrical appliances.

How and what the voltage measure

The voltage is measured using the device called a voltmeter. The voltmeter is connected in parallel to the element of the electrical circuit, where they want to measure the voltage drop. Denotes in the voltmeter schemes in the form of a circle, with the letter V.

Fig. 3. Various voltmeters and their designation in the schemes.

Previously, all the voltmeters were shooter, and the voltage value showed the arrow on the instrument scale with the applied digital values. Now most of these devices are available with electronic indication (LED or liquid crystal). The voltmeter itself should not affect the measurement result, so its own resistance is done very large so that the charges (electric current) are practically not taken through it.

What did we know?

So, we learned that the electrical stress is a physical value that characterizes the operation of the power of the electric field to move electrical charges. Voltage can be permanent or variable. Voltmeters are used to measure voltage.

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In fact, this term denotes the potential difference, and the voltage unit is volt. Volt is the surname of a scientist who marked the beginning of everything that we now know about electricity. And the name of this man Alessandro.

But this is what the electric current concerns, i.e. that with which household electrical appliances for us work. But there is also the concept of a mechanical parameter. Such a parameter is measured in Pascal. But it's not about him now.

What is equal to Volt

This parameter can be both constant and variable. Just an alternating current and "flowing" to apartments, buildings and structures, houses and organizations. The electrical voltage is amplitude waves, indicated on graphs in the form of sinusoids.

Alternating current is indicated in the circuits of the "~" icon. And if we talk about what is equal to one volt, it can be said that this is an electrical action in the chain, where the charge equal to one pendant (CL) is carried out, a job equal to one Joule (J).

The standard formula for which you can calculate it is:

U \u003d A: Q, where u is just the desired value; "A" is the work that the electric field performs (in j), transferring the charge, but "q" is just the charge itself, in the coulters.

If we talk about constant values, they practically do not differ from variables (with the exception of the construction schedule) and are also produced from them, by means of a rectifying diode bridge. Diodes, not passing the current into one of the parties, as if they divide the sinusoid, removing the half-wave of it. As a result, instead of phase and zero, it turns out plus and minus, but the calculus remains in the same volts (V or V).

Voltage measurement

Previously, only analog voltmeter was used to measure such a parameter. Now on the shelves of electrical stores, there is a very wide range of such devices already in digital performance, as well as multimeters, both analog and digital, with which the so-called voltage is measured. A similar device can be measured not only the magnitude, but also the strength of the current, the resistance of the chain, and even the ability to check the capacitor capacity or measure the temperature.

Of course, analog voltmeters and multimeters do not give such accuracy as digital, on the display of which the unit of voltage is highlighted up to hundredths or thousandth fractions.

When measuring this parameter, the voltmeter is turned on in the chain parallel, i.e. If necessary, measure the value between the phase and zero, the probes are applied to one to the first wire, and the other to the second, in contrast to the measurement of the current force, where the device is turned on in the circuit sequentially.

In the voltmeter schemes is indicated by the letter V, circled around. Different types of similar devices are measured, in addition to Volta, different voltage units. In general, it is measured in the following units: Millivolt, Microvolt, Kilovolt or Megavolt.

Voltage value

The value of this parameter of the electric current in our life is very high, because whether it matches it laid down, it depends on how much the incandescent lamp will be bright, and if compact fluorescent are installed, then the question arises, there will be or not, they will not burn at all. From his jumps depends on the durability of all light and domestic electrical appliances, and therefore the presence of a voltmeter house or a multimeter, as well as the ability to use them becomes necessary in our time.