Operating Bells at a Distance.
When it is desired to ring a bell situated at a considerable distance from the push, the resistance of the line becomes objectionable.
Fig. 15
On lines of 500 feet, No. 18 copper wire and upwards, the battery necessary would be very large, two small batteries and a relay would prove more satisfactory.
In [Fig. 15] the circuit of a simple form of relay is given. An adjustable contact screw C is placed where an extension S of the armature A can strike it. This extension is provided with a platinum contact. The connections are as in the figure.
When the push P is depressed, the current from the main battery M energizes the electromagnet E, and the armature A being attracted, contacts S and C meet. These contacts close the second circuit containing the bell B and the local battery L.
The relay resembles a second push near the bell, but controlled by current from a distance instead of being depressed by hand. Its advantage consists in it needing but a very weak current to move the armature A, which is held back by a light spring, or by gravity.
The relay may then be set near the bell and the wires from the push may be of a very great length. Battery L, which actually rings the bell, will thus only have to work through a few feet of wire.