As one might expect, there are instruments that can measure the power
delivered by a source and to a dissipative element. One such instrument,
the wattmeter, appears in Fig. 1.
Fig. 1: Wattmeter.
Since power is a function of both
the current and the voltage levels, four terminals must be connected as
shown in Fig. 2 to measure the power to the resistor R.
Fig. 2: Wattmeter connections.
If the current coils (CC) and potential coils (PC) of the wattmeter are
connected as shown in Fig. 2, there will be an up-scale reading on
the wattmeter. A reversal of either coil will result in a below-zero indication. Three voltage terminals may be available on the voltage side to
permit a choice of voltage levels. On most wattmeters, the current terminals are physically larger than the voltage terminals to provide safety
and to ensure a solid connection.