In an alternating current, the direction of the electric current reverses periodically, so that electrons oscillate inside the conductor at a frequency measured in number of cycles per second (or hertz, Hz). This Demonstration assumes there are no inductors or capacitors and that the impedance of the circuit is equal to the resistance of the incandescent light bulb. Then Ohm's law for the alternating current is given by , where is the voltage, is the resistance, and is the current. The power of the circuit is or , where is the root mean square value of the oscillating voltage, equal to , and likewise for . Power is measured in ampere-volts or watts (W).