Electricity and Magnetism

Using Variable Resistors

In the circuit below, notice that only two of the connections to the variable resistor are used.

The maximum resistance of the variable resistor is 100W.

When the sliding contact, S, is moved to A the voltmeter will read 6V (it is connected directly to both sides of the supply). This is, of course the maximum reading of voltage in this circuit.

What is the reading of the voltmeter when the sliding contact is moved to B?

We have, in effect, the following situation.

Therefore, the voltmeter will read 3V.

It is not possible to make the reading of the voltmeter vary from zero up to the full voltage of the supply using this circuit.

Variable Resistor used as a Variable Potential Divider

When we use all three connections of the variable resistor as shown above, the voltmeter reading can be reduced to zero by moving the sliding contact to B. The wire "x" (assumed to have zero resistance) is in parallel with the 100W resistor.

This circuit is useful in experiments in which we need a variable voltage supply.