# Test Instruments and Measuring Devices

Posted on Feb 5, 2014

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 (Figure 1). 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.

Equation (14-3) is the mathematical representation of the current without the meter installed. 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. Equation (14-5) is the mathematical representation for solving for the accuracy of the ammeter (KA). The percent loading error is that percent of error due to loading effects that result from the added resistance of the meter. Equation (14-7) is a mathematical representation of the percent loading error. A second error which occurs in an ammeter is calibration error. Calibration error is an error that occurs due to inaccurately marked meter faces. Typical values of calibration error in terms of full scale current are about 3 percent. Example: An ammeter, with a 10 mA full scale deflection and an internal resistance of 400 ©, is placed in a circuit with a 20 V power source and a 2 K © resistor (Figure 2). An ammeter with a full scale Im can be shunted with a resistor RSH in order to measure currents in excess of Im. The reason for shunting an ammeter is to extend the range of the ammeter and, thereby, measure currents higher than the original full scale value. Therefore, the input resistance of a shunted ammeter is related to the meter and shunt resistance. Equation (14-8) is a mathematical representation of this relationship. The purpose of a circuit breaker is to break the circuit...

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