Harper's Electricity Book for Boys

by Joseph H. Adams

A simple detector is made by winding fifteen or twenty feet of cotton-insulated copper wire No. 26 or 28 around the lower end of a drinking-glass. Leave six inches of each end loose; then after slipping the coil from the glass, tie the wires with thread at least four times around the circle, so as to bind the wires together. Press two sides of the hoop together so as to flatten it; then with paraffine attach the coil to a square block of wood, as shown in [Fig. 2].

From a thin clock-spring, not more than three-eighths of an inch wide, cut a piece two inches and a half long, and with a stout pair of tin-shears cut the ends so as to point them, as shown in [Fig. 3] A. With two pair of pliers bend a hump at the middle of the strip on the dotted lines shown in A, so that a side-view will appear like B in [Fig. 3]. Turn this strip over on a hard-wood block or a piece of lead, and with a stout steel-wire nail and a hammer dent the inverted V at the middle so that it will rest on the top of a needle-point without falling off.

With three little pieces of wood make a bridge and attach it to the wooden base over the paraffine that holds the wire-coil, and drive a needle down in the middle of it, taking care that it does not go through the back and touch the wires underneath. On this needle hang the strip of steel spring, and, if it does not properly balance, trim it with the shears or a hard file until it is adjusted properly. Rub this piece of steel over the pole ends of a large horseshoe magnet, or place it within the helix of a large coil and turn a powerful current through the coil. This will magnetize the strip of steel, which will then become a magnetic needle and hold the magnetism. Attach two binding-posts to corners of the block, and make the loose ends of the coil-wires fast to them. You now have a current-detector, or galvanoscope, as shown in [Fig. 4]. Turn the block so that the needle points to north and south and parallel to the strands of wire.

When the conductors from the poles of a battery or dynamo are made fast to the binding-posts, the needle will fly around to a position at right angles to that which it first occupied, as shown by the dotted line A A in [Fig. 4]. When the connection is broken the needle will turn around again and point to north and south, since the magnetic field about the wire ceases and disappears the instant the circuit is broken.

This is one of the strange and unknown phenomena of electricity, for while the current exerts a force that deflects the needle, it does not destroy its magnetism. On the breaking of the contact, no matter how long it may have held the needle away from its true course, it again points to north, and its magnetism is not affected.

Another simple current-detector is shown in [Fig. 5]. A piece of broomstick is sawed in half and both pieces are made fast to a block which is mounted on a base of wood three-quarters of an inch in thickness. The vertical block should measure five inches long, three inches high, and five-eighths of an inch thick. The half-circular pieces of wood are mounted so that the flat surfaces are three inches apart and the lower edges are one inch above the base-block. These may be held in place with glue and screws driven through the back of the vertical block and into the ends of the projecting half-circular pieces. The base-block is six inches long and four inches wide, and the vertical block is mounted on it one inch from an edge. The pieces of broomstick are two inches long, and at the front ends a thin bar of brass or copper is screwed fast to hold them apart and in proper position, as shown at A in [Fig. 5]. To improve the appearance of this mounting, all the wood-work may be stained and shellacked before the metal parts are attached.

With No. 26, 28, or 30 cotton-insulated wire make from fifteen to twenty wraps about the middle of the half-circular pieces of wood and carry the ends down through small holes in the base-block and thence through grooves cut at the under side of the block to the front corners, where they are to be made fast to binding-posts. A needle is to be set in the base-block midway between the two pieces of half-circular wood and through the strands of wire. Great care must be taken that the needle does not touch any bare wires, and as a precautionary measure it would be well to wrap the needle with a piece of insulating-tape where it passes through the strands of wire. Now place on the top of it a magnetized piece of steel, as described for the detector shown in [Fig. 4]. As it may not always be convenient to turn the instrument so that the needle points north, a small bar of magnetized steel or a stout needle that has been magnetized with a horseshoe magnet or a helix, may be laid across the half-circular wood pieces, so that it is parallel with the top layer of wires—in fact, it should rest on top of them.

By means of this needle, or bar, the magnetic piece of steel balanced on the vertical needle between the upper and lower strands of insulated wire may be held in one position no matter which way the block is turned. When the current passes in through one binding-post and out through the other (having thus travelled through the coil on the half-circular blocks) the needle is deflected and points out at the brass bar and back at the upright block.