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Why voltmeter is connected in parallel and ammeter in series?

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 Why the Voltmeter is Connected in Parallel and the Ammeter in Series | Explained with Illustrations

Measurement principle:

Ideally, a measuring instrument must not alter the reading being measured.

Loading effect of measuring instruments: 

What does it mean? E.g. a voltmeter must not load the (voltage) source to alter the actual reading.

Loading effect of voltmeter

The above circuit shows how a reading by the voltmeter is altered due to the loading effect as the input impedance of the voltmeter comes in parallel to the load.

Open circuit voltage measurement: 

Open circuit voltage measurement

Another example is measuring the open-circuit voltage with a voltmeter. The internal resistance of the battery or voltage source and the input impedance of the voltmeter form a voltage divider. That will decrease the actual voltage being measured.

Why a voltmeter must have a high input impedance: 

The input impedance of the voltmeter becomes critical for the voltage sources that have high internal impedance. For example, various process sensors. Better use a modern DMM that has FET input stages for very high input impedance.

Our practical voltmeter and ammeters are not ideal. We try to minimise reading errors by keeping the input impedance of the voltmeter as high as possible and the impedance of the ammeter as low as possible.

Practical Voltmeter:

Voltmeter internal impedance

Practical Ammeter:

Ammeter with shunt

Similarly, an ammeter (shunt) must be of low impedance (typically in milliohms). Because it comes in series with the load, which will decrease the actual current being measured.

Why a voltmeter in parallel and an ammeter in series?

Modern digital voltmeter typical input impedance is more than 10 MOhm. If we connect a voltmeter in series, a large input impedance will block the current passed to the load. So it will be pointless to do so.

It is obvious that ammeters, if connected in parallel, will create a short circuit.

In short, the Voltmeter must be parallel and the ammeter in series to the load.



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FAQ Set:

Q1. Why should a voltmeter be connected in parallel?

A voltmeter measures voltage across a component, so it must be in parallel to experience the same potential difference.

Q2. Why should an ammeter be connected in series?

An ammeter measures current through a component; a series connection ensures the same current passes through both.

Q3. What happens if we connect a voltmeter in series?

It will block most of the current due to its high resistance, disturbing the circuit operation.

Q4. What happens if we connect an ammeter in parallel?

It may short-circuit the circuit due to its very low resistance, causing damage.

Q5. What is the resistance of a voltmeter and an ammeter?

Voltmeter – very high resistance; Ammeter – very low resistance.


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