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.
Reducing Resistance of a Bell.
Sometimes it is desired to reduce the resistance the bell coils offer to the current, the bell then working over a very short line with few cells of battery. Or the bell coils may have been wound with fine wire for large battery voltage and a long line.
The bell coils may be put in multiple, the current then dividing and one-half going through each spool.
Untwist the joint between the spools near the yoke or iron bar to which the spools are attached. Join one of these ends to the wire at the armature end of the other spool and the second untwisted end to the armature end wire of its neighboring spool. Use short pieces of insulated wire for these extra connections.
The current now instead of having to go through one spool and then the other, can branch through both at once.
The resistance to the current of one spool is half the resistance of two, the current through one spool will therefore be twice that through the two spools as at first connected. And as there are two paths for it, each one-half the first resistance, the total will be only one-fourth the resistance of the ordinary series arrangement.
The same size battery will therefore send four times the current through the spools in multiple than when they are in series.
It is to be noted that the wire on one spool is wound in the reverse direction to that on the other. The reason will be apparent if the two spools and yoke are considered as merely one spool bent in a U or horseshoe form.