Page 28 - 12-phy-14 ELECTROMAGNETISM

14. Electromagnetism eLearn.Punjab

Thus zero of the scale is marked as infinity (Fig. 14 21-b). Now
a known resistance R is connected across the terminals c and

d. The galvanometer deflects to some intermediate point. This

(b) point is calibrated as R. In this way the whole scale is calibrated
Fig 14.21 A moving coil galvanometer is into resistance. The resistance to be measured is connected
converted into an ohmmeter. across the terminals c and d. The deflection on the calibrated

scale reads the value of the resistance directly.

14.11 AVO METER -MULTIMETER

It is an instrument which can measure current in amperes, potential
difference in volts and resistance in ohms. It basically consists

of a sensitive moving coil galvanometer which is converted into
a multirange ammeter, voltmeter or ohmmeter accordingly as

a current measuring circuit or a voltage measuring circuit or a
resistance measuring circuit is connected with the galvanometer

with the help of a switch known as function switch (Fig. 14.22).
Here X, Y are the main terminals of the AVO meter which are

connected with the circuit in which measurement is required. FS
is the function selector switch which connects the galvanometer

with relevant measuring circuit. Fig. 14.22

Voltage Measuring Part of AVO Meter

The voltage measuring part of the AVO meter is actually a
multirange voltmeter. It consists of a number of resistances

each of which can be connected in series with the moving coil
galvanometer with the help of a switch called the range switch

(Fig. 14.23). The value of each resistance depends upon the
range of the voltmeter which it controls.

Alternating voltages are also measured by AVO meter. AC
voltage is first converted into DC voltage by using diode as
Fig. 14.23
rectifier and then measured as usual.

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