_=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. [Illustration: Fig. 75.] =EXPERIMENT 106. To study the construction and use of a simple astatic needle.= _Apparatus._ Two unmagnetized sewing-needles (No. 1); horseshoe magnet, H M (No. 16); piece of stiff paper doubled and cut as in Fig. 75; a pin-point on which to support the paper. The pin may be stuck through a cork, or that of O C (No. 18) may be used. =250. Directions.= (A) Draw each needle across the N pole of H M five times from point to head (Exp. 9). This should make them of nearly equal strength, both points being N poles. (B) Stick the needles through the paper as shown, the N poles being at the same end of the paper. Balance the paper upon the pin-point. Has this combination a strong or weak pointing-power? (C) Turn one of the needles end for end. Again test the pointing-power. _=251. Discussion; Astatic Needles.=_ By arranging the needles so that their poles oppose each other, the pointing-power becomes almost nothing. This sort of a needle is needed in some experiments in electricity. Their magnetic fields are still retained. The combination is called an _astatic needle_; it is used to detect very feeble currents. The more nearly equal the magnets are in strength, the better. They are usually arranged with one above the other (Fig. 76). [Illustration: Fig. 76.] [Illustration: Fig. 77.] =EXPERIMENT 107. To study the construction and use of a simple astatic galvanoscope.= _Apparatus._ An astatic galvanoscope, A G (No. 59) (§ 252-254); dry cell, D C (No. 51); current reverser, C R (No. 57) (§ 235); wires for connections (§ 226).