﻿ Measuring apparatuses of electric units

# Types of measuring apparatuses

Measuring apparatuses are means by means of which invisible action of electrons can be registered and measured. Measuring apparatuses are necessary at the research of work of the chain. There are two types of measuring apparatuses. The first are the analog instruments using the graduated scale with the arrow. Other type — the digital instruments showing the size of counting of indications in the form of digits.

Classification of analog measuring apparatuses.

It is easier to read indications of digital instruments, and they provide big accuracy, than analog. However analog instruments  provide the opportunity to trace fast changings of current and tension.
The majority of measuring apparatuses is placed in the protective body. Conclusions are intended for connection of devices to the chain. For the correct connection of the device it is necessary to pay attention to polarity of conclusions. Color or white conclusions are positive, and the black conclusion — negative (or "earth").

Before use of the analog instrument of his shooter it has to be established on zero. On the obverse of any device there is the small screw by means of which installation on zero is made. Having established the arrow on zero, place the device where it has to be used. If the arrow is not necessary on zero, use the screw-driver for turn of the screw. The device should not be connected to the chain until installation of the arrow on zero is carried out.

## Measurement of current

Scheme of the instrumentation.

To use the ampermeter for measurement of current, the chain has to be opened, and the measuring device is inserted consistently into the chain.

At turning on of the ampermeter in the chain the polarity has to be observed. Two conclusions on the ampermeter are marked: positive — red, and negative (general) — black.

Caution: always switch-off the power source before connection of the ampermeter to the chain.

Negative conclusion has to be connected to more negative (with the smaller potential) the chain point, and the positive conclusion to more positive (with high potential) the chain point. After connection of the ampermeter of his shooter will move from left to right. If the arrow moves in the opposite direction, trade conclusions places.

Caution: the ampermeter never has to be connected parallel to any element of the chain. If to connect it in parallel, then the crossing point in the device will melt and will seriously damage the device or the chain. Never connect the ampermeter directly to the current source.

After installation of the ampermeter in the chain and before inclusion of the power supply install the device on the highest limit of measurement. After power supply inclusion the scale of the ampermeter can be switched to the most suitable. It will prevent the sharp movement of the arrow of the device to the right against the stop that can put out of action the device frame, namely the mechanism spring.

Internal resistance of the ampermeter increases to resistance of the chain and increases the general resistance of the chain. The measured circuit current can be lower, than the current flowing for lack of the ampermeter. However as resistance of the ampermeter is not enough in comparison with chain resistance, the mistake can be neglected.

The ampermeter with clips (clamp-on meters) does not demand connection to the measured chain. The ampermeter with clips uses the electromagnetic field created by current for measurement of size of the circuit current.

## Strain measurement

Permanent-magnet ammeter.

Tension exists between two points, it does not flow through the chain like current. Therefore, the voltmeter used for strain measurement is connected parallel to the chain.

Caution: if to turn on the voltmeter in the chain consistently, via it big current can go and damage it.

The polarity is here too important. The negative conclusion of the voltmeter has to be connected to more negative point of the chain (with the smaller potential), and the positive conclusion — to more positive point of the chain (with high potential). If points of connection to trade places, the arrow of the device will deviate to the left, and measurement cannot be taken. If it happens, trade places conclusions.

For carrying out measurements it is necessary to power off at first the chain, to connect the voltmeter, and then again to include the power supply. At first set the highest limit of measurement of the voltmeter. After tension is attached to the chain, establish the most suitable measuring scale of the device.

Internal resistance of the voltmeter is connected in parallel to the measured chain element. The general resistance to resistors connected in parallel is always less, than resistance of the smallest resistor. As a result tension which shows the voltmeter is, less, than actual tension for lack of the voltmeter.

In most cases the internal resistance of the voltmeter rather high and the mistake is so small that it can be neglected. However if tension is measured in the chain with the high resistance, resistance of the measuring device can give noticeable effect. Some voltmeters intended for such purposes have ultrahigh internal resistance.

## Measurement of resistance

The generalized block diagram of the analog measuring instrument.

The main assignment of the ohmmeter — measurement of resistance. Therefore, the ohmmeter can be used for definition what the chain is: opened, short-circuited go closed. The broken circuit has infinitely big resistance as through it current does not flow. The short-circuited chain has the zero resistance as current, passing through it, does not cause voltage drop. The closed circuit represents the full way for the current flow. Its resistance depends on resistance of components of the chain.

When component resistance is measured in the chain, disconnect one end of the component from the chain. It eliminates parallel ways which can lead to the wrong measurement of resistance. For receiving precise measurement the device has to be removed from the chain. After that conclusions of the ohmmeter are connected to the device.

Caution: Before connection of the ohmmeter to the chain be convinced that the power supply is switched off.

Check of the chain on isolation, open condition or the zakorochennost is called check of the chain on continuity. This check shows whether the way for current is continuous. To define, the closed circuit or is opened, the smallest sensitivity of the scale of the ohmmeter has to be used. At first be convinced that in the chain there are no components which can be damaged by current from the ohmmeter. After that connect ohmmeter conclusions to points of the measured chain. If the ohmmeter shows something, then the chain is closed or short-circuited. If the ohmmeter shows nothing (the arrow does not deviate) — the chain is opened. This check is useful to establishment of the reason for which the chain does not work.