The Screw Lamp Socket: Fitting a Bulb Makes the Circuit

"In a system of electric lighting where the lamps used consist of an incandescing conductor hermetically sealed in a glass inclosing globe it is desirable that the lamp and its supporting-socket should be so constructed that the lamp may be readily removed from or placed in the socket, and that at the same time by the act of placing in or removing the lamp from the socket the appropriate circuit connections to the conductor be formed, subject, however, and only, to the control of a suitable circuit-controller". The socket is of insulating material, hollowed to receive the neck at one end and "formed with a screw-thread at the other end, by which it may be attached to any ordinary gas-fixture, chandelier, or bracket." "Upon the interior of the socket is a metal band, which may be formed into a female screw-thread", joined to one conductor; "One or two turns of the screw-thread is sufficient". When the lamp is in place one neck band touches the socket's band, and the other is in position to be touched by the circuit-controller. Instead of a thread, a concave band held by curved spring fingers is also described.

Use only low voltage: a screw-base 12 V lamp and socket, a 12 V battery or power supply, and a switch. With the multimeter, trace which of the socket's two terminals goes to the threaded shell and which to the centre contact. Wire the switch into the centre-contact lead, fit the lamp and switch it on. Loosen the lamp half a turn until it flickers; measure the voltage across the loose contact while it is lit and compare with the notebook's heating. Never do this on mains wiring.

**Thomas A. Edison** of Menlo Park, New Jersey, assignor to the **Edison Electric Light Company**, filed US 251,554 on 26 March 1881; it is dated 27 December 1881. **Honest limits.** Thread size, friction, lamp current and contact resistances in the notebook are examples; the patent gives no dimensions.