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.