APPARATUS 108.
_159. Automatic Interrupter._ An ordinary electric bell, or buzzer, may be used as an interrupter. Every time the vibrating armature swings, the circuit is opened. The combination of a battery, induction coil, and electric bell makes a very good outfit for medical purposes. The automatic interrupter used on App. 100 should be studied.
CHAPTER XIII.
CURRENT DETECTORS AND GALVANOMETERS.
_160. Current Detectors; Galvanometers._ When a wire carrying a current of sufficient strength is properly brought near a magnetic needle, the latter will be deflected from its N and S line. The conducting wire has a magnetic field while the current pa.s.ses through it, and this gives the wire the power to act upon a magnetic needle just as another magnet would.
The action of detectors, etc., depends upon this fact; and, strange to say, the magnetic field about the wire disappears the instant the current ceases to pa.s.s. The combination, thus, of a coil of wire and a magnetic needle, properly arranged, makes an instrument with which the presence of electricity can be detected. When the strength of a current is to be measured, or the strengths of two currents are to be compared, the apparatus is called a galvanometer. The method of making these pieces of apparatus will depend upon the strength of current to be tested or measured.
APPARATUS 109.
_161. Current Detector._ Figs. 38 and 40 show magnetic needles. These may be used to detect a current by holding the conducting wire near them and parallel to the needle. This form is not sensitive to weak currents.
The delicacy of the apparatus is increased by allowing the wire to pa.s.s above and below the needle several times as in the next apparatus.
APPARATUS 110.
_162. Current Detector._ Fig. 83 consists, like all detectors, of a coil and a magnetic needle. The other parts are merely for convenience. Each turn of the coil helps to move the needle when the current pa.s.ses.
[Ill.u.s.tration: Fig. 83.]
163. The Coil is made by winding 10 feet of No. 30 insulated copper wire around the end of a broom-handle or other cylinder that is about 1 inch in diameter. This length of wire makes about 32 turns around such a cylinder. The exact length of wire for this makes no difference. After winding it, the coil should be slipped from the handle, being careful to hold it in such a way that it cannot uncoil and spring away from you.
Tie the coil together with thread, in 3 or 4 places, to keep it in shape, and leave 5 or 6 in. of wire free at each end, so that connections can be made with other pieces of apparatus. After this is done press the coil into the shape shown, Fig. 83. This brings the wire near the needle and allows a longer needle to be used. The coil may be fastened to a pasteboard base. To do this, p.r.i.c.k 4 holes in the base near the ends of the oval coil, and pa.s.s a strong thread through these with the aid of a sewing-needle. Tie the thread on the underside of the base at each end. If this is well done, the coil will be held firmly in an upright position. Paraffine may be used instead of the thread.
The ends of the wire should be made bare, and these may be sewed to the base to keep them in place.
164. The Needle may be supported upon a pin or needle-point. The piece of needle should be stuck through a cork which has a slot cut into its underside, so that it will straddle the lower part of the coil. The height of the needle-point should be fixed so that the horizontal ends of the magnetic needle will be near the axis of the coil, that is, along its central line.
165. To Use the Detector, turn its base around until the coil is in the N and S line--that is, until the magnetic needle is parallel to the length of the coil and wholly inside of it. Touch the ends of the coil with the two ends of the wire, which is supposed to carry a current. The needle will fly around until it is nearly perpendicular to its former position, if the current is strong enough.
APPARATUS 111.
[Ill.u.s.tration: Fig. 84.]
_166. Current Detector._ Fig. 84. To make a more substantial detector than App. 110, the coil should be fastened to a wooden base. The coil may be made of 10 ft. No. 30 wire, as explained. (-- 163.) A hole should be made in the base with a small awl or with a hot wire, and into this should be set a pin, head down. The hole need not be larger than the pin-head, and when you find out how high the pin-point should be above the base, the pin may be fastened in place with a little paraffine, which should be pressed into the hole around the pin. The coil may be fastened in place with paraffine. The ends of the coil may be connected with binding-posts, described in App. 46, as shown, or with any other desired form.
The base should be 4 5 7/8 inches. The coil looks well when placed about 1 in. from the edge of the base. The binding-posts may be about 1 in. from the edges.
APPARATUS 112.
_167. Current Detector._ Fig. 85. This is more troublesome to make than App. 111, but perhaps it looks more scientific.
168. The Coil is wound around 2 ordinary spools which are glued to a vertical piece, which, in turn, is screwed to a base. You should not use iron nails or screws in the construction of electrical apparatus, when a magnetic needle is to be used in connection with it, as these would attract the needle. The spools may be pushed onto dowels which are fastened into the vertical piece. Small bra.s.s screws are good for the purpose also, if you haven"t good glue or the dowels. This coil, etc., may be used in connection with an astatic needle. The coil may be wound with App. 93 or 94, if you make the attachment of App. 95, and screw the upright carrying the spools to the attachment.
[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.