Question #3260c

Feb 17, 2016

Current certainly does flow through a voltmeter. However because the voltmeter is connected in parallel and has a high resistance, this current will be very small. This is in order to prevent it disturbing the circuit that it's measuring, too much.

Explanation:

Current certainly does flow through a voltmeter. However because the voltmeter is connected in parallel and has a high resistance, this current will be very small. This is in order to prevent it disturbing the circuit that it's measuring, too much.

Only a very small current is needed because inside the voltmeter is an instrument called a galvanometer. This uses a coil to deflect the voltmeter's needle, when subjected to a magnetic field generated by the current. It is very sensitive, so only a tiny current is needed to actuate it. Confusingly, the galvanometer is actually measuring the current passing through it, but because the resistance is known, this can be converted into a voltage using Ohm's law: V = I R.

In contrast an ammeter is connected in series with a circuit and so must have a very low resistance, to prevent a large voltage drop across it.

Kirchhoff's first law tell us that currents split when components are connected in parallel. Thus the some of the current will branch off into the voltmeter, which is connected in parallel. However, the resistance is high in the voltmeter, so only a small current will branch off. This current is so small that sometimes people wrongly think that there's no current passing through the voltmeter.

Note that I have described an analogue voltmeter here. Digital voltmeters work on the same principle but use electronics to measure the voltage rather than a galvanometer.