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.
Chapter VI
CURRENT-DETECTORS AND GALVANOMETERS
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.
The action of detectors depends upon two things—first, the magnetized needle that, when properly balanced, will point north and south; and, secondly, a current of electricity passing through a wire or wires held above the needle, or both above and below it. This is more clearly shown in [Fig. 1], where a compass is resting on a wire connected to a battery. The wire also passes over the top of the compass, which doubles the electro-magnetic field.
When the compass (with the needle pointing north) is resting on the wire that is attached to the zinc pole of a battery, and when the end of the wire that passes back over the top of the compass is attached to the carbon pole, the needle will fly around and point to the east. When the wires are reversed, the needle will point to the west. Thus the combination of a battery or other source of electric current, a magnetic needle, and a coil of wire properly arranged, make an instrument that will detect electric currents and may be correctly called a current-detector. The pressure of more or less current is determined by instruments wound with wire of different sizes; the finer the wire the more sensitive the instrument, and consequently the more delicate. A very weak current can only be detected with a delicate and sensitive instrument. The coarser the wire and the larger the instrument, the better it will be for testing strong currents that would perhaps burn out the fine wire of the more delicate apparatus.
This instrument, when placed between a source of electricity and a piece of apparatus, such as a bell, a motor, or lamp, does not weaken the current, for it requires no waste of electricity to operate the magnetic needle. Consequently, when a very weak current is being used for any tests, it is well to place a detector between the battery and the apparatus to show that the current is actually passing through the wire.