The standard test circuit is used for testing components and plotting graphs of voltage against current. It consists of the component to be investigated in series with an cell of known voltage, ammeter and a variable resistance and in parallel with a voltmeter.
As the resistance
is varied the total resistance in the circuit is varies,
and the current is then
The voltage across the component is
By varyingtherefore, both the voltage across the component and the current through it can be varied, and a set of voltages and corresponding currents through the cell obtained. Typical Curves for a diode, resistor and a filament lamp are obained below (negative values are obtained by turning the component around).
