Calibration of Ammeter

The practical set up for calibration of Ammeter is as shown in figure.

Coarse
⋅ Ammeter to be
calibrated

Fine
adjustment

Calibration of ammeter

An ammeter to be calibrated is connected in series with a standard resistance R, of
suitable value, The current supplied by a D.C. supply passes through ammeter as well as
standard resistance R,. Using d.c. potentiometer, a voltage across standard resistance can
be measured, Thus, current flowing through R, (and ammeter also) is given by,
ಗೆ

I = रि,
where V, = voltage across R, measured using potentiometer
R, = resistance of standard resistor

Since the resistance of standard resistance is known accurately and also the voltage
across R, is measured using standardised potentiometer, the method of calibrating
ammeter is very accurate. If the actual current ‏ہی1‎ and the current indicated by ammeter
1 are not matching, error is indicated. The percentage error is given by,

% error = ‏بر سھگ ھا‎ 100
ind
where 1.3 = Current indicated by ammeter and
327
laa = 1७ = ज

The calibration curve of ammeter can be obtained by plotting % error against the
reading of ammeter i.e. ಓಲ.

Advantages of Maxwell Bridge
The advantages of the Maxwell bridge are,
1) The balance equation is independent of losses associated with inductance.
2) The balance cquation is independent of frequency of measurement.
3) The scale of the resistance can be calibrated to read the inductance directly.
4) The scale of R, can be calibrated to read the Q value directly.

5) When the bridge is balanced, the only component in series with coil under test is
resistance R,. If R, is selected such that it can carry high current, then heavy
current carrying capacity coils can be tested using this bridge.

Disadvantages of Maxwell Bridge

The disadvantages of the Maxwell bridge are,

1) It cannot be used for the measurement of high Q values. Its use is limited to the
measurement of low Q values from 1 to 10. This can be proved from phase .angle
balance condition which says that sum of the angles of one pair of opposite arms
must be equal.

6, +0, = 92 +63