Chapter 13

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 passes 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 pass. 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 pass 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 passes.

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, prick 4 holes in the base near the ends of the oval coil, and pass 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.

_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. (Sec. 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 x 5 x 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 brass 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.

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 passing 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 x 4 x 5/8 in. The upright piece is 5 x 3-1/2 x 5/8 in. The spools are 2-1/2 in. apart center to center.

APPARATUS 113.

_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 x 5 x 7/8 in. The coil is made from 10 ft. of No. 30 insulated copper wire. (See Sec. 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 brass 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.

_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 Sec. 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 (Sec. 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 brass 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.

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 pass 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.

179. The Base should be about 7 x 5 x 7/8 in. Fasten three bent brass or copper strips to the base with brass screws to hold the chimney steady. By bending them in more or less you can make a snug fit around the chimney.

180. Adjusting the Needle. In the center of the box-cover is a small hole. The thread from the needle passes through this. The upper end of the thread is wound around a screw-eye, which is screwed into the cover near one edge. By turning the cover around, the needle can be made to hang parallel to the coils, and by turning the screw-eye, the needle can be raised or lowered. A small hole should be made in the cover before putting in the screw-eye, or you will be liable to split the wood.

181. Use. This apparatus will indicate very slight currents; in fact, as feeble ones as the student will have occasion to experiment with, such as induced currents, currents of thermo-electricity, and currents produced by exceedingly weak batteries. (See text-book.)

APPARATUS 116.

_182. Tangent Galvanometer._ Fig. 90. For the uses of this form of galvanometer see text-book. Do not use any iron in making this apparatus. The base is 5 x 4 x 7/8 in. At its front end are three binding-posts. The pasteboard band, G, is 1-1/4 in. wide and 6 in. in diameter. Cut the pasteboard 21 in. long and 1-1/4 in. wide, then bend it into the form of a circle. There will be a lap of about 3 in., and you can make it solid by sewing the two ends together at the lap.

183. The Coils maybe made of No. 24 insulated copper wire, which should be wound on before fastening G to the base. There are two separate coils, one having five turns and the other ten turns. Leaving a 6-in. length, A, for connections, wind five turns of wire on to G, putting them on clockwise; that is, pass them over the top of G from left to right. Tie thread around G and the wire to hold them together after you have five turns on, and cut a 6-in. end, B. Now begin with C, and wind on ten turns, bringing the end of them out at D. Punch holes, F, through G on each side of the coils, run twine, T, through them, and tie T on the outside of G. Do this in three or four places, to firmly hold the coils.

184. Fastening Coils to Base. The band and coils will not rest squarely upon the base, so cut two pieces of wood, E, about 2 x 1/4 x 1/4 in., to be put under G, one being on each side of the coil. Make holes through the base, pass strong cord, H, through them, and over the inside of G, then tie under the base. This should tightly squeeze E, and hold G upright and firm.

185. The Connections. A and B are the ends of the five-turn coil; C and D are the ends of the ten-turn coil. If the battery-wires are connected with X and Y, the current will pass through five turns of wire; if connected with Y and Z, it will pass through ten turns; if with X and Z, the current will pass through the entire fifteen turns. In this way the strength of the magnetic field about the coil can be regulated, and its effect upon the magnetic needle, M, changed.

186. To Support the Needle, glue or sew two strips, I, to G. They must be in such a position that the poles of M will be as nearly as possible in a horizontal line drawn through the center of the circle, G. After you have made M (App. 66), and have found where the pieces, I, should be, fasten them to G, and then to I glue a pasteboard strip, J, about 1-1/4 in. wide. Run a pin, P, up through the center of J to support M.

187. The Magnetic Needle, M, should not be over 1 in. long for this kind of an instrument. (See App. 66 for full directions for making it.) On the top of M should be fastened a light paper pointer or index, L. The short end should be made large, so that the long slim end will not over-turn M; that is, the pointer should balance itself. It may be fastened to M with paraffine or a drop of sealing-wax. If carefully balanced, the pointer can be made quite long.

188. The Graduated Circle, K, is described. (Index.) With this you can tell through how many degrees the needle is deflected, when the current passes. The strength of different currents can be compared, and many interesting experiments performed with the tangent galvanometer. For clearness, the circle, K, is shown small. In order to have the divisions on it far enough apart, K should be about 4 in. in diameter. The zero points should be at the front and back of the instrument, when a pointer is used on the needle.

189. How to Use It. For full explanations, and for the study of experimental cells, etc., by means of the tangent galvanometer, see text-book. It will be impossible for you to get M exactly in the center of G; you cannot get the pointer exactly at right angles with M; hence, if you pass a certain current through the coils, and the pointer reads 20 degrees, you will find, if you reverse the current, making it go through the coil in an opposite direction, that the pointer may read 24 degrees on the opposite side of the zero. To get the true reading, then, take the average of the two, which in the case mentioned would be 22 degrees. (See current reversers.)

APPARATUS 117.

_190. Tangent Galvanometer._ Fig. 91. The base consists of 2 parts, A and B. It is not necessary to use two pieces if you have wood that is at least 7/8 in. thick. This is given as a suggestion in case you have nothing but thin boards. By screwing B to A the base is made thick enough to take the screws for binding-posts. The base proper, A, is 8-1/2 x 5 x 1/2 in. If you make this of 7/8 in. stuff, you will not need B.

The Back, C, is 10 x 8-1/2 x 1/2 in. It is screwed to the base. Do not use nails, as these affect the magnetic needle. Find the center of C, and with this as a center, draw two circles, (that is, the circumferences of two circles,) one 5 in. in diameter to show where to cut out a hole, H, and the other 7 in. in diameter to serve as a guide for fastening on the spools, F.

The Spools, F, are glued to C. If you have brass screws, these may be used instead of the spools; they should be left sticking out from C about 1 in. Around the spools or screws, fasten a pasteboard band, G, on which to wind the wire. G may be about 1 in. wide; it should be kept in the circular form by sewing the ends together where they lap. (Read directions in App. 116.)

191. The Coils on this model are 4 in number. (See App. 116 for the method of winding.) The first coil is made of coarse wire, No. 18, its ends being joined to the binding-posts, V and W. The second coil has 5 turns of No. 24 insulated copper wire, its ends being joined to W and X. The third coil has 10 turns of the same size wire, No. 24, and is joined to X and Y. The fourth coil has 20 turns of the same joined to Y and Z. If you want to use the galvanometer for quite weak currents, it would be well to make a fifth coil of 20 turns of No. 30 wire, and join it with Z and a new binding-post. The ends of the coils are run through small screw-eyes before passing to X, Y, etc. This is not necessary, it merely keeps them in place.

The Binding-Posts are like App. 43. Any other desired style may be used, those of App. 46 being preferred.

The Hole, H, is 5 in. in diameter. It should be cut out about 1/2 in. below the center of the circles to allow for D, and for the pin-point which supports the magnetic needle, the poles of which should be in the line passing through the center of the coils. The method of cutting the hole, H, through C, will depend upon the tools at your service.

D is the front edge of an adjustable table, like that explained. (Index.) It is 4-1/4 in. wide. It supports the magnetic needle which is inside of E.

E is the outside of a glass-covered compass. (See App. 67 for details.) The needle should not be over 1 in. long.