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7. The Transformer

The transformer depends for its operation on the principle of
mutual induction. The primary winding of the transformer is
connected to the power source (which must be a AC) and the load
is connected to the secondary winding.
The transformer may have either a single winding (when it is
known as an autotransformer) or more than one winding (two
winding transformers are the most common single phase
transformers). The iron circuit of the transformer is laminated to
reduce the eddy-current power-loss. Important rules relating to
transformer design are:
1) Each winding supports the same number of volts per turn.
2) Ampere-turn balance is maintained between the windings.The
efficiency of a transformer is the ratio of the power it delivers to
the load to the power absorbed by the primary winding.

8. Measuring Devices
Ammeters and Voltmeters

Ammeters measure the current flowing in a circuit and
normally have scales, which are graduated or calibrated in
amperes, milliamperes or microamperes.
Voltmeters are used to measure the potential difference
between two points in a circuit. The calibration of voltmeters is
usually in volts, millivolts and microvolts.
The main difference between the two instruments of the
same type or design is in the resistance of the operating coil
identical moving units may be used for either meter. An ammeter
is connected in the positive or negative lead in series with a circuit
and, therefore, must have a low resistance coil otherwise the
readings would be incorrect as the coil would absorb an appre-
ciable amount of power.
A voltmeter is connected in parallel across the points of a
circuit where the difference of potential is to be measured. The
resistance of the operating coil must, in this instance, be as high as

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