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Connecting a voltmeter in series

You don't want to change what you're trying to measure

When you make a measurement you don't want your measuring instrument to change what you're actually trying to measure.  There'd be no point in tying a great heavy wheel to a sprinter to find out how fast he was running.

Voltmeters are connected in parallel because they have to sample a difference in voltage between two points.

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So do we want our voltmeter to have a very high resistance or a very low resistance?  The answer may surprise you.

Voltmeters have a very high resistance

A combination of a high resistance (like a voltmeter) connected in parallel with a smaller resistance (like a bulb) has an effective resistance of a tiny bit less than the small resistance (the bulb).  In other words the effect is pretty much the same as just having the bulb alone, which is what we want.

If the voltmeter had a very low resistance then the effective resistance would be a little less than the voltmeter itself.  This would be much less than just the bulb on its own so by adding the voltmeter you'd be changing the circuit you were trying to measure.

The problem with connecting a voltmeter in series

If you break a circuit and insert a voltmeter then you're introducing a big resistance into the circuit and so the current is small everywhere.  This means the bulb will be out, which means you're not measuring the voltage across a bulb when it's lit.

Using a voltmeter in open circuit

Confusingly the voltmeter will still show a reading of 6 volts because nearly all the voltage drops across the voltmeter, rather than the bulb.

If you removed the voltmeter so the circuit wasn't complete anymore then the voltage is still there, even though no current flows.  So connecting a voltmeter like this is really like measuring the voltage between two points in 'open circuit'.

back to Lesson 5: Voltage and Current