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.
Wattmeter connectionsWattmeter connections
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.

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