Electric circuit. Capacitor (blue, lower centre) connected to an alternating current (AC) power supply (orange, left). This type of supply operates with the electrical current flowing back and forth at a certain frequency. In the UK, the current alternates 50 times per second. An ammeter (centre) is measuring the flow of the electric current around the circuit in amperes, while a voltmeter (right) is measuring the voltage (volts), or potential difference, in the circuit. The voltmeter is showing a negative reading due to the reverse flow of the current.