Part 11 (2/2)

[Ill.u.s.tration: Fig. 85.]

The binding-posts, shown in Fig. 85, are not to be advised. It will be better to use those of App. 45. The magnetic needle is supported by a sewing-needle stuck through a cork. This may be fastened to the base with paraffine.

_169._ It is often troublesome to turn the apparatus around until the needle becomes parallel to the length of the coil. To avoid this, a small bar magnet, shown in the Fig. 85, may be laid on top of the coil.

A magnetized sewing-needle will do, and this will keep the magnetic needle quiet and parallel to it when the current is not pa.s.sing through the coil. Of course, it takes a little more current to move the magnetic needle when the bar magnet is in place, than it does without the magnet.

_170._ By allowing the current to enter the right-hand binding-post, as you look at it from the front (Fig. 85), it will go around the coil in the direction of the hands of a clock, that is, from left to right on top. This, of course, is not necessary to merely detect the presence of a current. In order, however, to determine the direction of currents by means of a magnetic needle, study the effect with a single turn of wire at first. (See text-book.)

171. Dimensions. The base is 5 4 5/8 in. The upright piece is 5 3-1/2 5/8 in. The spools are 2-1/2 in. apart center to center.

APPARATUS 113.

[Ill.u.s.tration: Fig. 86.]

_172. Astatic Current Detector._ Fig. 86. The ordinary magnetic needle points to the north quite strongly. It is evident, then, that this pointing-power must be overcome by the magnetic field around the coil of wire, before the needle can be forced from the N and S line. Very weak currents will not visibly move the magnetic needle in the detectors so far described. You should remember that no action will take place unless the magnetic field around the magnetic needle is acted upon by that around the coil. In order to make an instrument that will be very sensitive, we must have strong fields about the needle and coil, and we must, at the same time, decrease the pointing-power of the needle. We can increase the strength of the field about the needle, and at the same time decrease its pointing-power by using an astatic needle. (See App.

69.) The arrangement shown in Fig. 86 is a very simple one, and it is quite sensitive.

173. Details of Construction. The base is 4 5 7/8 in. The coil is made from 10 ft. of No. 30 insulated copper wire. (See -- 163 for details about coil making.) The binding-posts are like App. 41. The Astatic Needle is described for App. 69. The needles may be broken off, if too long for the coil. They are supported by a fine thread hung from a screw-eye, which may be turned to adjust the position of the needles.

This is not necessary, as the thread may be hung from a plain wire arm that reaches out from the upright rod. This rod is a 6-in. piece of dowel, 1/4 or 5/16 in. in diameter. It stands in an ordinary spool which should be glued to the base. Do not nail it to the base. The wire arm may be of iron, as it is some distance above the needle; but it is better to use a stiff bra.s.s or copper one. In the figure one end of the wire is twisted around the screw-eye, making a nut for the screw-eye to turn in.

Hang the astatic needle so that the wire between the two parts will not quite touch the coil. The needles should be parallel to the coil before testing for currents. They will fly around very decidedly with even fairly weak currents.

APPARATUS 114.

[Ill.u.s.tration: Fig. 87.]

_174. Astatic Current Detector._ Fig. 87. For a description of the wood-work, coil, etc., see App. 112; for the astatic needle see App. 69; for the method of supporting the needle see App. 113, Fig. 86. The top part of the coil is spread apart a little to allow the lower needle to be dropped through the opening thus made, and to allow the wire joining the two needles to be free to turn. The needles may be broken off a little, if necessary, or an opening may be cut into the vertical part of the frame, so that they can swing more freely. This detector will indicate quite feeble currents.

APPARATUS 115.

_175. Astatic Detector._ Fig. 88. As previously Stated, the sensitiveness of a detector can be made greater by increasing the strength of the coil-field for a given current. This may be done by increasing the number of turns of wire in the coil. The most convenient way will be to use two coils, one on each side of the astatic needle.

176. The Support, or framework, is a lamp chimney. By this the astatic needle is suspended and protected from air currents. The chimney should be at least 3 in. in diameter at the bottom, about 10 in. high, with a plain round top. Upon the top of the chimney is placed the cover of a wooden pill-box, 2 in. in diameter.

177. The Coils should be made separately, for convenience. Each should be of 10 ft. No. 30 wire. (See details -- 163.) Cut out a round piece of stiff pasteboard, just large enough to go inside of the bottom of the chimney. Fasten the coils to this by sewing (-- 163), or with paraffine, so that they shall be symmetrically located and 3/8 in. apart. The pasteboard circle may be fastened to the base with small bra.s.s screws.

Do not use any iron nails or tacks. In this, all four ends of wire are brought out under the edge of the chimney (Fig. 88). Cut little grooves in the base for the wire to sink into, so that the chimney will rest firmly upon the base all around. The ends of the wires are fastened to three binding-posts.

[Ill.u.s.tration: Fig. 88.]

178. Joining the Coils. The end of one coil must be joined to the beginning of the other properly, or the action of one will destroy that of the other. Fig. 89 shows the two coils, A and B. If the current enters at the binding-post, X, it will pa.s.s through the turns of coil A, in the direction of clock-hands, then out to Y, where B begins, around B in the same way, and then to Z. Y may be simply a screw-eye binding-post (App. 41). By this arrangement one or both coils can be used at a time. If the current is very weak, use both coils; that is, connect the ends of wires to be tested with the two outside binding-posts. If they are joined to the middle and one outside post, one coil only will be in the circuit.

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