Ammeter loading effect and How it is Compenstated

An ammeter is an electrical instrument which measures electrical current.

You always follow two steps for measuring current:
  1. Disconnect the circuit
  2. Connect the ammeter in series
However, the internal resistance of ammeter impacts the current as well.

Let's consider a 5 V source that is connected to a 1 kΩ.
From Ohm's law, the current passing through the circuit is 5 mA. (I = V/R)

Let's hook an ammeter having 0.2 kΩ internal resistance in series.
The overall current in circuit is now 4.16 mA. (I = V/R = 5 V / (0.2 + 1) = 5 V / 1.2 k = 4.16 mA)

The ammeter will display 4.16 mA whereas the actual current is 5 mA. This difference of actual and measured current is technically termed as loading effect of an ammeter.
Let's reconsider the above example with an ammeter of 0.01 kΩ.

The current I is now 4.95 mA. (I = V/R = 5 V / 1.01 kΩ = 4.95 mA
The loading effect now reduced from 16.8% to 1.2%.

I = (5 mA - 4.94 mA)/5 mA = 1.2%

We observe that with a lower internalresistancee the loading effect reduced from 16.8% to 1.2%.

That is what it happens in practical ammeters. Practical meters are designed to keep the internal resistance as much smaller as it is possible.


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