Learn the basics of Electrical Engineering.

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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