English: A measuring Leyden jar, used in 19th century electrostatic experiments to measure electric charge. The device was connected between an electrostatic machine that produced high voltage static electricity, and a load. The high voltage would charge the Leyden jar capacitor up until the voltage equalled the breakdown voltage of the spark gap. Then a spark across the gap would completely discharge the jar. Since the jar held a fixed amount of charge ${\displaystyle Q=CV}$ at the gap's breakdown voltage ${\displaystyle V}$ each spark transferred the same amount of charge. So the total amount of charge could be measured by counting the sparks. An electrostatic machine's output current could be measured by the number of sparks per minute.