English: An antique battery charger from 1922 for charging storage batteries used to run vacuum tube radios. Semiconductor and vacuum tube diodes had not been developed yet, and the device uses mechanical vibrating contacts to rectify the 60 Hz house current to provide the DC needed to charge the battery. An electromagnet with a permanent magnet core (center) is used to close switch contacts (right front) to make a half-wave rectifier. During half the AC cycle, the magnetic field of the electromagnet and the permanent magnet oppose each other, so the magnet produces no net magnetic field, so the contacts stay open and no current is allowed through to the battery. During the other half of the cycle, the electromagnet and permanent magnet's fields are in the same direction and aid each other, pulling the iron armature of the contacts over, closing the contacts, so current flows through a current-limiting resistance to the battery, charging it. The switch contacts are tuned to resonate mechanically at 60 Hz, the frequency of the house current, aiding the action. The charger has an ammeter on the front to monitor the rate of charge.
This model was the "F-F Radio Rectifier" made by the France Mfg. Co., Cleveland, Ohio, USA, and was rated at 6A output to charge 6V lead-acid batteries and cost $15.