In this post, we are going to discuss different ways of measuring the resistance of a component. Resistance is defined as R = V/I. This obviously gives ways of measuring resistance, as in the circuits shown below. Each of the first 2 methods listed below uses both an ammeter and a voltmeter. And, the third method uses a multimeter.
first method to measure the resistance of a component
This method requires two meters and a power supply, together with a calculation, to find the resistance.
Refer to figure 1. The current, I, in the resistor is measured by the ammeter, the potential difference, V, across the resistor is measured with the voltmeter, and then R = V/I.
A small amount of current is taken by the voltmeter, so the current recorded by the ammeter is not, strictly speaking, the actual current in the resistor. As long as the voltmeter has high resistance (or much greater than that of the resistor) we can ignore this effect.
second method to measure the resistance of a component
Like the first method, this method also requires two meters and a power supply, together with a calculation, to find the resistance.
Ammeters should have a very small resistance so that they do not affect the current that they are measuring. Nevertheless, the ammeter might have a useful effect on the circuit of Figure 2, particularly if the resistor also has a low resistance.
The ammeter does, indeed, measure the current in the resistor, but now the voltmeter measures the p.d. across both the resistor and the ammeter. In most cases, the circuit shown in figure 1 seems preferable when methods 1 and 2 are compared.
Digital ohmmeter to measure resistance
A quicker and easier way is to use a digital ohmmeter. As its name suggests, this instrument measures ‘ohms’ – that is, resistance – directly.
Most digital multimeters have an ‘ohms’ range. Alternatively, digital ohmmeters can be purchased as separate instruments.
It is important to understand that a digital ohmmeter does not actually measure resistance directly in the way that an ammeter measures current.
When we turn the dial on a multimeter to the ‘ohms’ range, we bring a battery into play, which produces a very small current in the component whose resistance is being determined. This current is measured by the meter and then converted into a resistance reading by the ‘electronics’ inside the meter.
Note: Warning! Remember, an ohmmeter produces a current. It should therefore never be used to measure the resistance of a component that is connected to a circuit or else damage may be done to the circuit or the ohmmeter.