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.