1-22
For higher current ranges (above 50 amperes) ammeters that use external shunts are used. The
external shunt resistor serves the same purpose as the internal shunt resistor. The external shunt is
connected in series with the circuit to be measured and in parallel with the ammeter. This shunts
(bypasses) the ammeter so only a portion of the current goes through the meter. Each external shunt will
be marked with the maximum current value that the ammeter will measure when that shunt is used. Figure
1-23 shows an ammeter that is designed to use external shunts and a d’Arsonval meter movement. Figure
1-23(A) shows the internal construction of the meter and the way in which the external shunt is connected
to the meter and to the circuit being measured. Figure 1-23(C) shows some typical external shunts.
Figure 1-23.—An ammeter employing the d'Arsonval principle and external shunts.
A shunt resistor is nothing more than a resistor in parallel with the meter movement. To measure
high currents, very small resistance shunts are used so the majority of the current will go through the
shunt. Since the total resistance of a parallel circuit (the meter movement and shunt resistor) is always less
than the resistance of the smallest resistor, as an ammeter’s range is increased, its resistance decreases.
This is important because the load resistance of high-current circuits is smaller than the load resistance of
low-current circuits. To obtain accurate measurements, it is necessary that the ammeter resistance be
much less than the load resistance, since the ammeter is connected in series with the load.
Q20. What electrical property does an ammeter measure?
