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THE RELAY COIL Remember we mentioned a coil in the relay having many turns of wire around an iron core? We’re going to use a coil similar to this in a fun project on the secret electromagnetic lock. What is an electromagnetic lock? It’s a vertical coil around a nonmagnetic tube (instead of the iron core in the telegraph relay) with an iron rod suspended in the center. The rod is supported by a piece of elastic thread and is arranged so that part of it extends above the tube. It serves the same purpose as a sliding bolt in an ordinary lock. With the magnetic lock mounted on, say, the in- side of a drawer near the top, the protruding rod prevents the drawer from being opened. But when a pair of secret terminals on the outside of the drawer are bridged, the coil yanks the rod down, thus allowing the drawer to be pulled out. When the circuit is broken, the rod will be lifted to its original position by the elastic thread. Power for the electromagnetic lock is supplied by a 6-volt battery kept inside the drawer.
Experiment 3A Secret Drawer Lock |
| Materials: 1/4” nonmagnetic tubing 2-1/2” long. 35’ of hook- up wire. An 8-penny nail. Elastic thread. Metal strip. 4 small nails. 2 machine screws with nuts. 6-volt battery. |