Simple Ammeter Review

The ammeter measures electric current. It may be calibrated in amperes, milliamperes, or microamperes. In order to measure current, the ammeter must be placed in series with the circuit to be tested.

When an ammeter is placed in series with a circuit, it will increase the resistance of that circuit by an amount equal to the internal resistance of the meter Rm. The equation below is the mathematical representation of the current without the meter installed.

Io = V/Ro

The equation below is the mathematical representation of the current with the meter installed in the
circuit.

Iw = V / [ V / (Ro + Rin) ]

The accuracy of the ammeter KA is the ratio of the current when the meter is in the circuit, Iw, to the current with the meter out of the circuit, Io. The equation below is the mathematical representation for solving for the accuracy of the ammeter (KA).

KA = Iw/Io

By substitution laws, the equation below is a mathematical representation of the accuracy using circuit resistance.

KA = [ V/(Ro + Rm)] [ Ro/V ] - Ro/(Ro + Rm)