Science: Silent Mercury

When a customer enters a telephone booth and closes the door, a switch located in the ceiling and connected to the door by a rod turns on the light. In some booths it starts a ventilating fan. Switches heretofore used for this purpose were mechanical ones which had moving parts to wear out and which made what Bell Telephone Laboratories considered a disagreeable noise.

Described in Bell Laboratories Record last week was a booth switch in the form of a small glass tube containing a pellet of mercury, which is a good conductor of electricity. When the door is open the mercury remains at one end of the tube. Closing the door tilts it so that the mercury rolls to the other end, closing a circuit between two electrodes, lighting the light and starting the fan. This little switch makes no noise, has no moving parts.†