Generally, an electrical current may be measured by using a shunt. A shunt is a resistor of known resistance, which is placed in series with a load so that all of the current to be measured flows through the resistor. The voltage drop across the resistor is proportional to the current flowing through resistor. Since the resistance of the resistor is known, a voltmeter connected across the resistor can display the current value.
One disadvantage of shunts is their high resistance and therefore their loss of energy.