247. Directions. (A) Arrange as in [Fig. 74], the coil being N and S ([§ 244]). Join the ends of the wires, 2 and 3, with the 5-turn coil of G V as shown. Wire, 2, is connected to L ([Fig. 72]). Press lever 2 of C R ([Fig. 69]) for an instant, watching the compass-needle and noting how many degrees it swings the first time. Get thoroughly in mind the direction in which the N pole of the needle is deflected when the current passes around G C in a "clockwise" direction. There must be no magnets, iron, or pieces of steel within 3 feet of A G.
(B) Press lever, 3, for an instant, watching the needle. The current will now pass in an "anti-clockwise" direction. Is the needle deflected about the same number of degrees as in (A)?
(C) Change the ends of the wires, 2 and 3, to the 10-turn coil ([§ 242]) and repeat (A) and (B).
(D) Change 2 and 3 to L and R ([Fig. 72]), thus allowing the current to pass around 15 turns; then repeat (A) and (B).
248. Discussion; True Readings. Is not possible to get the magnetic needle, M, exactly in the center of G C; the pointers will not exactly be in the axis of M; the coils will not be exactly N and S: hence, if you pass a certain current through the coil and the pointer reads 20 degrees, you will find, if you reverse the current, that the pointer may read 24 degrees on the other side of the zero mark. To get the true reading, average the two, in this case the average being 22 degrees.
The galvanoscope gives us an instrument with which we can study, more fully, cells, currents, etc.
249. Note of Caution. It has already been stated that the compass-needle should be in the center of the coil ([§ 243]), and that the coil should be in the N and S line ([§ 244]). In addition to the above, see that there are no magnets near G V, when using it; tap G V occasionally to be sure that the needle swings freely, hold the eye directly over the pointers when reading degrees; the pointers should be at zero when no current passes through G V; be sure that the electrical connections are good.
There are several sources of error in taking readings, and in all the quantitative experiments given. The author takes it for granted that such errors will be looked out for by the teacher.
