=231. Directions.= (A) Stand the compass and D C near each other (Fig. 66). Attach one end of an insulated copper wire, C W, to the binding-post, C, which is on the carbon plate of the cell. Do _not_ join the other end to the other binding-post, Zn, of the zinc plate. (B) With the left hand hold C W above and near the compass-needle, and in the N and S line, so that it will extend over the entire length of the needle. (C) Take the free end of C W in the right hand and touch binding-post, Zn, for an instant only, watching the needle. Repeat. _=232. Current Detectors.=_ We know that a magnet can act, by induction, through the air upon a piece of iron or upon another magnet. The deflection of the needle in this experiment shows that there must be a magnetic field around a wire carrying a current. This fact is of the greatest possible importance. The simple magnetic needle, when used as above, becomes a detector of electricity. [Illustration: Fig. 66.] [Illustration: Fig. 67.] =EXPERIMENT 102. To study the construction and use of a simple "key."= _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. [Illustration: Fig. 68.] =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.