_=237. Discussion.=_ The reverser gives us a quick and easy means of reversing the current which is to pass through any desired instrument, first in one direction and then in the opposite direction. Suppose (Fig. 69) that the current enters C R at C, as it does when C is joined to the carbon of the cell; the current can go no farther until one lever is lowered. If lever 2 (Fig. 69) be now pressed down, as in part (A), the current will pass along 2, which does not now touch 4, out through X to a coil of wire or any instrument, and back to the reverser by the wire joined to Y. It will then pass from 3 onto 4, to Z, and back to the Cell; that is, the current enters C W at X. When lever 3 is pressed, the current still entering C R at C, the electricity will pass onto 3 and out at Y, and back through X, 4 and Z to the cell. The current, then, can be made to pass out of X or Y at will by pressing the proper lever. This experiment also teaches something about currents, but these will be discussed later. [Illustration: Fig. 71.] =EXPERIMENT 104. To study the simple current detector.= _Apparatus._ The compass (No. 18); dry cell, D C (No. 51); current reverser, C R (No. 57); copper wire, C W, a few feet long, with spring connectors on its ends. (See Apparatus Book, Chapter XIII, for Home-Made Detectors.) =238. Directions.= (A) Join the ends of the wire to X and Y of the reverser, C R, as in the last experiment. Coil up C W so that you can hold the coil with your left hand, as shown in Fig. 71, the magnetic needle being inside of it and parallel to it. (B) Press lever 2 of the reverser for an instant only. Is the needle deflected more or less than it was when the wire simply passed over or under it once? (C) Reverse the current through C W by pressing lever 3, and note the result. (D) Get clearly in mind which way the N pole of the needle is deflected when the current enters C W at X, also when it enters at Y. _=239. Discussion.=_ The current passed _over_ the needle in one direction, and _under_ it in the opposite direction; that is, the part of the wire above _helps_ that below. Each turn of the wire increases the strength of the magnetic field about the coil, and helps to deflect the needle. In this way, by increasing the number of turns, detectors may be made that will show the presence of very weak currents. The magnetic fields about wires and coils will be studied in a later chapter. [Illustration: Fig. 72.] =EXPERIMENT 105. To study the construction and use of the simple galvanoscope.= _Apparatus._ The galvanoscope, G V, complete (No. 58), described in § 240-246; dry cell, D C (No. 51); current reverser, C R (No. 57) (§ 235); wires, with spring connectors, to join the different pieces of apparatus (§226). (See Apparatus Book, Chapter XIII, for Home-Made Galvanoscopes.)