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Potential divider
A potential divider is a circuit that gives a variable output potential difference from a supply. Output voltage (V2) = (R2/[R1 + R2])V

Replacing R2 with a Light dependent resistor (LDR)
If the LDR is connected as part of a potential divider as shown in the diagram then as the light level is increased its resistance falls and the proportion of the input voltage dropped across it will also fall. So in the light V2 is low and in the dark V2 is high.

Replacing R2 with a thermistor
if R2 is replaced by a thermistor. As the temperature of the thermistor rises its resistance falls and so the voltage dropped across it falls.
When the thermistor is hot V2 is low and when the thermistor is cold V2 is high.

Measuring the output
(a) Using a digital voltmeter with a very high (if not virtually infinite) resistance. The output voltage (VO) measured by the meter really is that across R2, in other words V2.
(b) Using a moving coil meter a much lower resistance than a digital meter.
The proportion of the input voltage (V) dropped across R2 therefore falls and so the output voltage (VO) is less than that measured with a digital meter.
© Keith Gibbs 2008