Measuring Resistance and Power

Simple ammeter/voltmeter circuit

Circuit diagram: Battery, resistor and ammeter in series, voltmeter across the resistor.

R is anything with resistance. This could be a bulb, fixed value resistor, length of resistance wire, etc. The rheostat is used to change the current in the circuit. The ammeter measures the current. The voltmeter measures the potential difference (voltage) across the resistance R.

	symbol	quantity	unit	unit symbol
	V	potential difference (voltage)	volt	V
	I	current	amp or ampere	A
calculated quantities
R = V/I	R	resistance	ohm	Ω
P = I×V	P	power	watt	W

Practical Notes

Rheostat

Before the circuit is completed the rheostat must be adjusted to its maximum resistance. This allows the current to be increased as the resistance of the rheostat is decreased. If you start with the rheostat at zero resistance there is a possibility that the initial current may too high resulting in damage to the component being measured. E.g. A bulb could blow or a piece of resistance wire could melt.

Even with the rheostat at maximum resistance if the voltage of the battery or power supply is too high you could still damage the component. If in doubt, start with a very small voltage as well as maximum resistance on the rheostat.

For this type of circuit to work effectively the resistance of the rheostat must be larger than R to be able to change the current significantly. Typical lab rheostats are usually designed to cope with largish currents and so have a low resistance (e.g. 10-20Ω). You should find the resistance and maximum current rating marked on the rheostat. For large values of R the rheostat can be replaced with a variable resistor that has a resistance in the appropriate range.

Voltage source

For larger voltages replace the cell with a DC power supply. For small currents and voltages cells have the advantage of producing a steady voltage. A mains operated power supply has to first convert the alternating mains supply (AC) to direct current (DC) and then smooth it. There may still be a slight ripple to the DC voltage produced even after smoothing.

A variable voltage DC power supply does allow you to quickly and easily alter the voltage/current to the circuit in addition to the rheostat. Be careful, however, because a simple twist of the dial could easily destroy your component. Determine what the maximum safe voltage is that you can use, and make sure that this is not exceeded. Some power supplies have a locking mechanism you can set to make sure that a maximum voltage is not exceeded. It is always a good idea to use this feature to prevent possible accidents if it is available.

Circuit diagram: Variable voltage power supply and bulb.

This would be a suitable circuit to use with a 6 volt bulb. A variable voltage DC power supply removes the need to use a rheostat. You can run the bulb above its normal operating voltage of 6 volts, however this will reduce the lifetime of the bulb. If you go too high the filament will of course get too hot and melt. (12 volts would certainly blow the bulb!)

On/Off Switch

A switch is not shown in the circuit but you could easily add one. (If you are using a DC power supply it may have its own switch.) A push to make switch that automatically releases when you take your finger off can be a good idea. If you are taking measurements of resistance at constant temperature then you need to use a small current and leave the circuit switched on for as short a time as possible. This will reduce the heating effect of the current and the resulting change in temperature and resistance of the component. This obviously doesn't apply to something like a bulb, which is designed to heat up (and glow) as a result of the current passing through it.