In figure 1-21(C), the voltage is reduced from 100 volts to 50 volts. In this case,
Notice that the relationship (ratio) of IR1 and IR2 remains the same. IR2 is nine times greater than IR1
and IR1 has one-tenth of the total current.
If R1 is replaced by a meter movement that has 10 ohms of resistance and a sensitivity of 10
amperes, the reading of the meter will represent one-tenth of the current in the circuit and R
nine-tenths of the current. R2 is a SHUNT resistor because it diverts, or shunts, a portion of the current
from the meter movement (R1). By this method, a 10-ampere meter movement will measure current up to
100 amperes. By adding a second scale to the face of the meter, the current can be read directly.
By adding several shunt resistors in the meter case, with a switch to select the desired resistor, the
ammeter will be capable of measuring several different maximum current readings or ranges.
Most meter movements in use today have sensitivities of from 5 microamperes to 1 milliampere.
Figure 1-22 shows the circuit of meter switched to higher ranges, the shunt an ammeter that uses a meter
movement with a sensitivity of 100 microamperes and shunt resistors. This ammeter has five ranges (100
microamperes; 1, 10, and 100 milliamperes; 1 ampere) selected by a switch.