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.
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.)
240. The Galvanoscope ([Fig. 72]) is more than a mere detector of electricity. With it we shall be able to study, more fully, cells, currents, etc., etc. We must first understand its construction.
241. The Coil-support, C S, is fastened to the cross-piece, C P, on which are the 3 binding-posts or coil-ends, L, M and R (left, middle, right). The legs, G L, are screwed to C P in such a way that C P is held a little above the table: this allows C S to be tipped to the front or rear to adjust it vertically. On account of the peculiar arrangement of the legs, the galvanoscope can be made to stand firmly, even upon uneven surfaces. The screws holding G L should not be put in far enough to tear the threads in the wood, C P.
242. The Galvanoscope Coils, G C, are two in number, both being fastened to the coil-support, C S. The first coil has five turns of wire, its ends being fastened to L and M; the other coil has ten turns, with ends at M and R. The current can, at will, be made to pass through 5, 10 or 15 turns of wire by making the proper connections.
Suppose that we have two wires direct from a cell, or from the current reverser, with spring connectors on them so that we can slip them onto L, M or R, which stand for left, middle or right. When the wires are joined to L and M the current can pass through but 5 turns; when joined to M and R it will go through 10 turns; and when to L and R it will pass through the entire 15 turns. When the current enters the galvanoscope at L and passes out at M or R, it will pass through the turns of wire from left to right, at the top; that is, it will pass in a "clockwise" direction.