English: A Wehnelt interrupter, an antique electrical device invented in 1899 by German physicist Arthur Wehnelt and used with large induction coils until the 1920s. Induction coils require an interrupter in the primary circuit to periodically break the DC primary current to create the magnetic flux changes in the coil that induce voltage in the secondary coil. The Wehnelt interrupter consisted of a short platinum needle electrode projecting into a jar of electrolyte such as dilute sulfuric acid, and an anode electrode consisting of a lead plate. The above interrupter has three electrodes; the short platinum wires can be seen projecting out of the bottom of the white porcelain insulators. When the coil's primary current flowed through the interrupter, it continually created hydrogen gas bubbles on the platinum needle that repeatedly broke the circuit. The Wehnelt interrupter could create up to 2000 "breaks" per second, so it was superior to the vibrating arm "hammer" interrupters which were limited to a maximum rate of 200 breaks per second. It was used with the large induction coils that were employed to power cold-cathode x-ray machines and spark-gap radio transmitters up to the 1920s.