Variable resistors are still used (this written
2016) in various electronic devices, for example, as volume
controls in amplifiers, dimmer circuits for lights etc. 

They have, in many cases,
been replaced by digital circuits but some of the same
principles still apply. 




In this circuit, notice that only two of the connections of the variable
resistor are being used. 

In what follows, we will assume that the maximum resistance of the variable
resistor (that is, the resistance between points A and B) is 100W. 

When the sliding contact, S is moved to A, the voltmeter will read
6V. In this situation it will be connected
directly to both sides of the supply. 


However, what will be the reading of the voltmeter is S is moved to
B? 





When S is moved to B we have, in effect, this situation. 

Clearly, the voltmeter will read 3V (see
here if in doubt). 



We will now consider using all three connections of the variable
resistor to make a variable potential divider. 

With S connected to A, the situation is as before, therefore the voltmeter
reads 6V. 




However, with S moved to B, the wire x that we have added (assumed to have
zero resistance) is now in parallel with the 100W
resistor. 

This means that the voltmeter will now
read zero. 
(NB The circuit symbol for the variable
resistor is not usually drawn like this.) 

Hence a variable potential divider circuit is more useful as a supply of
variable voltage. 

However, it should be noted that a system like this does not provide a
stable variable voltage supply. 




If we connect only a voltmeter, as
shown here, we can predict its reading, knowing the position of
S. 
If we assume S is half way between A
and B, then V = 3V. 

Remember that, if we are using a suitable voltmeter, it will have a
resistance that is very much higher than the other components in
the circuit. 
In calculations, we often consider it
to have infinite resistance. 

If we now connect a component having a resistance of 50W
(for simplicity of calculation!) the
voltage will decrease significantly. 











This is because we now have, in effect, the circuit shown in the next
diagram. 





Two 50W
components in parallel have an effective resistance of 25W
so the voltage indicated by the voltmeter will now be one third of the
supply rather than one half as above, so V now equals
2V. 




Conclusion 

The voltage can still be varied from zero up to the voltage of the
supply but it also varies depending on what we
connect to the variable potential divider. 
