Measuring volts, amps and ohms

Multimeters, and relatively good ones at that, are now frequently available for under £20.  A little knowledge is required to use the meters successfully and as there’s a lot of DIY going on at the moment we thought it may be helpful to set out the basics.

Be very careful if doing any of this, particularly with ‘mains’ electricity – it can cause injury and death.

Volts, amps and ohms

The best analogy for these electrical terms is water. Think of a dam, a sluice gate and a river below it with some boulders in it.

The potential energy, the water behind the dam in the reservoir, is the equivalent of battery.

If you have a battery you measure it’s potential energy in volts.

With the water behind the dam and the sluice gate closed, no water flows. Open the sluice gate and water will flow at a rate down the river. The rate of flow is the equivalent of the rate of flow in an electrical circuit. We measure this in amps (short for amperes).

The rate the water flows down the river is determined by what prevents it flowing as fast as possible. There will be some resistance, let’s say boulders in the river. This is the equivalent of electrical resistance.

So how do we measure volts, amps and resistance (ohms). Lest assume a simple circuit with a battery, heating element, and switch.

The voltage is there whether we have any flow or not. So we measure this across the battery terminals. Your meter must be set to volts to do this, at a range higher than the expected voltage.

For a DC (direct current circuit. e.g. in a car) set the meter to suitable DC volts range, typically shown as ⎓.

For an AC (alternating circuit. e.g. household electricity) set the meter to suitable AC volts range, typically shown as a ‘wavy’ line.

Note that the leads go into the centre and right hand connection points unless current is being measured, in which case it’s the centre and left hand connection points.

If you were to measure the flow of current in a river you would have to put something in the river. Similarly to measure the electrical flow rate we need to insert the meter in the electrical circuit so it is in series – so you will need to disconnect a part of the circuit and insert the meter in it. You may need some temporary cabling to do this (you would only do this on a DC circuit e.g. a car and not on mains electricity). Set the meter to a DC or AC current range which is higher than the expected current. NEVER connect a meter set to amps across e.g. a battery, it will blow the fuse inside (if there is one) and potentially damage the meter irreparably.

To measure the resistance to the flow of water in our river you would probably find a way of measuring the size of the boulders. It’s the same with electricity. Just as you would try and measure a boulder in a river when the water was NOT flowing we don’t measure circuit resistance with the power  on.

The relationship between volts, amps, resistance and watts

There is a simple mathematical formula which determines the relationship between these basic electrical properties. It is called Ohms Law and is:

V = I x R    where V = volts,   I = current in amps and   R = resistance in ohms (symbol Ω)

This formula is good for DC circuits and can act as an approximation for AC circuits.

The other little formulas worth knowing is that watts (W) = V x I.  Watts is energy so for instance a traditional 60 watt light bulbs consumes 60 watts of energy.

There’s a simple way of demonstrating this and one that gets used a lot in testing immersion heaters in homes. An immersion heater is usually rated at 3KW = 3 kilowatts or 3000 watts.

3000 watts at 240 volts is 12.5 amps. Using Ohms law R = V/I = 240/12.5 = roughly 19 Ohms. This is what one would expect to see when measuring an immersion element; with the power off!