A mechanism developed by Matthaus Hipp to maintain a pendulum in oscillation electrically (also called 'Hipp's butterfly switch'). The pendulum rod carries a free-swinging vein or trailer which passes over a notched block mounted on the upper pair of contacts. When the pendulum amplitude falls sufficiently, the lower edge of the vein enters the notch, and on the return swing of the pendulum, the contacts close. Mounted on the lower end of the pendulum is an iron bar, which passes close to an electro-magnet, which is energised on closure of the contacts in a phase relationship to the pendulum, so as to maintain the pendulum's amplitude. Alternatively, the same system may be used to energize, whenever the amplitude of the pendulum falls, a coil mounted on the lower end of the pendulum, swinging over a fixed magnet.