Apparatus. A key, K (No. 55) ([§ 233]); a dry cell, D C, (No. 51); a compass, O C (No. 18). Arrange as shown in [Fig. 67], which is a top view or plan. Connect the pieces of apparatus with wires and spring connectors ([§ 226]). Binding-post, C, is joined to I (in) of the key; O (out) of key is joined to binding-post, Zn, the wire, C W, passing directly over and near O C, which is to be used as a detector. The current cannot pass until the lever, L, is pressed. A metal plate, M P, is used to connect two short wires ([§ 228]) in case C W is not long enough.
233. A key is merely a piece of apparatus by which the circuit can be conveniently and rapidly opened and closed at the will of the operator; that is, by it the electricity can be quickly turned on or off. [Fig. 68] shows a simple form of key. To the base, B, are fastened two metal pieces or straps, the upper one, L, being the lever or key proper. The front end of L is raised above O, so that the two do not touch each other unless L is firmly pressed down. A screw, S, keeps L from springing too far above O. For convenience we shall suppose that the wire leading to the key joins it at I (in); the wire from the key is joined to O (out), by means of connectors ([§ 226]).
The key may be put into any circuit by first cutting a wire and then joining the ends to I and O. Spring connectors make the best connections with this form of key. (For Home-Made Keys see Apparatus Book.)
234. Directions. (A) The magnetic needle being directly under the wire, press L down for an instant only and note the action of the needle.
(B) Press L again, hold it down for 3 seconds, not over that, and watch the needle.
Discussion. The key allows us to easily regulate the length of time during which the current passes. This experiment shows, also, that the magnetic field about the wire disappears as soon as the current ceases to pass.
