A simple way to mount the spools is shown at A in Fig. 15. A notch is cut in the face of the blocks large enough to admit the axle; then a face-plate is screwed over the end of the block to hold the axle in place. This arrangement makes it easy to remove the spool, in case of necessity, without detaching the arms from the cross-rail.
Two sets of wires are wound on the large spool, one leading to the right-hand and the other to the left-hand push-button on the table-rails. The ends of the wires are arranged so that one leading from both directions is made fast to one circular strip on the spool, the other two being attached to the remaining band. This is more clearly shown at B in Fig. 15, where the ends are visible as they are twisted together and pass through their respective holes. The spool is then turned over, and six or eight feet of wire wound on from each side. The spring is coiled up and caught on the nail or screw, and the ends of the wires are made fast to the binding-posts near the push-buttons. The wires from both push-buttons are then in connection with the circular bands, which in turn are connected to the bands on the smaller spool, and lead the current down through the floor wires. By pushing the button at either end the circuit is closed and the buzzer or bell is rung in the kitchen or pantry.
Arranged in this manner, the wires are kept off the floor, no matter where the table is moved, and it can be drawn open as wide as may be found necessary to put in leaves. When closed again, the spring causes the large spool to revolve and wind up the wire.
A current-detector is a necessary part of the amateur electrician’s equipment; technically, this piece of apparatus is called a galvanoscope.
When a wire or a number of them are brought near a magnetic needle or a small compass, the needle will be deflected from its north and south line and will point east and west, or west and east, according to the direction in which the current is passing through the wires. All wires that are conducting electricity have a magnetic field, and when brought near the magnetized needle of a compass they have the power to act on it the same as another and stronger magnet would.