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.

=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.

=243. The Compass-needle=, furnished with O C (No. 18), will do also for this galvanoscope. It should be placed upon the pin-point after fixing on the pointers (§ 246). The length of the needle should be parallel to the plane of the coil when no current passes; that is, the coil and coil-support should be in the N and S line.

The needle can be centered in regard to right and left, and in regard to up and down, by properly adjusting the position of the pin-point support, P P S; this is held firmly to C S by two spring-connectors. By removing S C, the support, P P S, may be raised or lowered.

=244. To place the coil= in the N and S line, simply swing the galvanoscope bodily around, at the same time looking down upon the needle, until the length of the needle becomes parallel to the coil-support. When once carefully adjusted N and S, a line may be drawn upon the table as a guide for its position in future experiments. The coil should stand in a vertical plane, and this straight up and down position can be easily adjusted.

To place the coil in the E and W line, turn it until the pointers are at the 90° (90 degree) marks,--the 0° (zero degree) marks remaining, of course, as described above.

=245. The Degree-Card, G D C= (Fig. 72) has a dot at its center, to show where to make a pin-hole for the pin that supports the compass-needle. With this you can tell how many degrees the needle is deflected when the current passes. This card, G D C, should be pressed down over the pin-point. The zero points of G D C should be N and S, also, when the coil is in that position; that is, they should be in the plane of the coil. The pointers on the needle (§ 246) will then be at O, when the needle is at rest, no current passing through the coils. (See Apparatus Book § 272 for Home-Made Degree-Card.) G D C may be held permanently in position after it is adjusted, by sticking a short pin through it into P P S. Do not let this pin interfere, however, with the swinging of the needle.

=246. Pointers= (Fig. 73) should be fastened to the needle, in order to make the readings of degrees accurate. Fasten to the compass-needle a piece of No. 30 insulated copper wire, as shown. It may be cut to the proper length after it is wound around the needle. See that the wire does not touch the pin when needle is in place; balance needle by cutting a little from the heavier end of wire with shears; bend the ends of wire so that they are at opposite sides of the degree-card, both pointing at O, for example. The needle must swing freely, be nicely balanced, and the wire must not touch pin or degree-card.

=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.

=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).

=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).

_Arrange_ as shown in Fig. 80, which is a top view. The wires from C R are connected to the binding-posts of A G at the back, the spring connectors being slipped into them (§ 229).

=252. Construction of the Astatic Galvanoscope.= When not to be used for a long time, or for shipping, the legs, A (Fig. 77) may be removed, and the whole packed inside of the box, B.

The _Coil_, C, has a resistance of about 5 ohms, and is fastened to the coil-support, C S. The ends of the coil are permanently fastened to the binding-posts, L and R (left and right). The ends are so arranged that when the current enters at L it will pass around the coil in a clockwise direction.

=253. The Astatic Needle= (Exp. 106) is supported by a small thread, T, which is tied to the thread-wire, T W. This T W springs into an eyelet, E, which, in turn, rests in a hole made in the end of B. E should turn easily in the hole, but it should not wabble.

Fig. 78 shows a sectional view of the coil and needle. The wire, W, should be bent, as shown, so that the magnets can be as near the center-line of C as possible. Fig. 79 shows a front view of the needle. As a matter of convenience it will be best to arrange the poles of the needles, as shown, to agree with the descriptions of the experiments.

To keep the needle from being affected by air currents, the glass plate (No. 38) may be placed in front of the box, B. Stand it upon the legs, A, and tie a string around it, and B, to hold it in place.

=254. Adjusting the Needle.= As T is tied to T W, the needle may be swung completely around by turning T W. This should be done until the length of the needle is parallel to the turns of C. The up and down position of the needle should be fixed as nearly as possible when fastening T to T W, the exact place being finally fixed by raising or lowering T W through E. The spring in T W should hold it firmly in E after adjustment. The wire, W, joining the needle-magnets should not touch the coil. It may be made to just swing free from C by tilting the box forward or backward a little. The construction of the legs, etc., makes it possible to tilt the box, and to make it stand firmly upon an irregular surface.

=255. Directions.= (A) See that there are no magnets within 3 feet of A G. Test the astatic needle, after you have it properly suspended, to convince yourself that it does not try to swing around in a N and S line. In case the needle-magnets have been in contact with other magnets, or are not equally magnetized, remagnetize them as directed in Exp. 106. They must remain in any position given them by turning T W. Finally, bring them parallel to the turns of the coil. (See § 254.) Arrange as in Fig. 80.

(B) Press lever 2 of C R (§ 235) for an instant only. This allows the current to enter A G at L. Repeat several times until you thoroughly fix in your mind the direction in which the right-hand end of the needle is deflected. Does the needle jump suddenly when the current passes?

(C) Press lever 3 for an instant only. Study the result.

_=256. Astatic Galvanoscopes.=_ It is evident that in the ordinary current detector (Exp. 104), the pointing power of the needle has to be overcome by the magnetic field about the coil, before the needle can be forced from its N and S line. Very weak currents will not visibly move the needle in ordinary detectors. To make a sensitive instrument we must have strong fields about both the needle and coil, and we must, at the same time, decrease the pointing power of the needle. Both of these things are accomplished by using an _astatic needle_ in connection with a coil containing considerable wire. The uses of the astatic galvanoscope will be studied more fully in later experiments.