Watt's law: Variable resistor, glow coil, volt and amp meter.

Watt's law: Variable resistor, glow coil, volt and amp meter. The electric heating element (glow coil) in this demo has a resistance of about 19 ohms. When the knife-switch is closed, a current flows through the coil, causing the coil to heat up. Watt's law states that the rate of energy transfer P equals the current times the voltage.