In principle, when used together, an ammeter and voltmeter allow for the measurement of the resistance of any circuit element. Let’s assume that that element is a simple ohmic resistor. Suppose that the ammeter is connected in series with the resistor (which is connected to an ideal power source with voltage V) and the voltmeter is placed across the resistor only.
(a) Sketch this circuit (with instruments connected) and use it to explain why the correct resistance is not given by R = V/I, where V is the voltmeter reading and I is the ammeter reading.
(b) Show that the actual resistance of the element is larger than the result in part (a) and is instead given by R = V / I – (V/Rv), where Rv is the resistance of the voltmeter.
(c) Show that the result in part (b) reduces to R = V/I for an ideal voltmeter.