# From galvanometer to ammeter

##### Class practical

Adding a shunt in parallel with a galvanometer converts it to an ammeter with a higher range. This is a trial and error method, not one involving calculations.

#### Apparatus and materials

For each student group

Galvanometer

Ammeter, 0 to 1 A, DC

Cell, 1.5 V in holder

Power supply, low voltage, DC

Crocodile clips, 2

Lamp in holder, 12 V, 36 W or 24 W

Eureka wire, 28 SWG or thicker

#### Procedure

Making an ammeter

a Your galvanometer is designed to measure small currents of a few milliamps. When the pointer is at the end of the scale, the current through the little coil which moves with the pointer must be, say, 0.01 A (or whatever your galvanometer is built to measure there). Suppose you wish to use it to measure much larger currents, say 1 A, at the end of its scale. The rest of that large current (1 A minus 0.01 A, for example) must travel by an alternative route a loop line in parallel.

b For that loop line or shunt, connect a short piece of alloy wire across the terminals of your galvanometer, as in the diagram. Take care! If, when adjusting the shunt, you let the whole big current go through the galvanometer, even momentarily, you might damage the galvanometer badly.

c Start with a very short shunt, straight across from terminal to terminal. Make a very rough test of that by connecting in series a lamp, your shunted galvanometer, a commercial ammeter (for comparison) and one 1.5-volt cell - just for a safe first trial.

d Switch on the current just for a moment, to see whether the pointer moves too far or too little.

e Adjust the length of shunt by trial and error. Shorten or lengthen the shunt until your home-made ammeter seems to read roughly what you want it to read.

f Disconnect the battery from your test circuit and replace it with the power supply, set to give 12 V. Adjust the shunt more carefully till you have a good ammeter.

g A commercial ammeter is constructed like this. It is a milliammeter with a shunt. Sometimes the basic instrument has several removable shunts to make it an ammeter with a choice of several ranges-as in the case of multimeters where you can select a range by turning a dial.

#### Teaching notes

1 Those who are keen to use their knowledge of resistance can have a go at converting a milliammeter to an ammeter. This is the job which a meter shunt does. The correct resistance has to be connected in parallel with the milliammeter in order to allow it to register amps. It does this by sending most of the circuit’s current through the shunt and tapping a small fraction of it to send through the meter.

2 The commercial ammeter enables students to adjust their home-made one to read as they wish, by a trial and error method rather than one in which the resistance of the shunt is calculated.

This experiment was safety-tested in October 2006