Electronic Electrical Devices  Electronic Electrical Components
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.
I_{o} = V/R_{o}
The equation below is the mathematical representation of the current with the meter installed in the
circuit.
Iw = V / [ V / (R_{o} + R_{in}) ]
The accuracy of the ammeter K_{A} is the ratio of the current when the meter is in the circuit, Iw,
to the current with the meter out of the circuit, I_{o}. The equation below is the mathematical
representation for solving for the accuracy of the ammeter (K_{A}).
K_{A} = I_{w}/I_{o}
By substitution laws, the equation below is a mathematical representation of the accuracy using
circuit resistance.
K_{A} = [ V/(R_{o} + R_{m})] [ R_{o}/V ]  R_{o}/(R_{o} + R_{m})
The percent loading error is that percent of error due to loading effects that result from the added
resistance of the meter. The equation below is a mathematical representation of the percent loading
error.
% Loading Error = ( 1  K_{A}) (100 %)
