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.
If both spools were wound in the same direction they would be in opposite directions when the U were straightened out, and would cause like poles at the same ends. These poles would neutralize one another, so that there would be no magnetic attraction.
This can be readily proved by joining together the two yoke ends and the two armature ends of the spool wires. Then pass the current through these two joined connections.
=The Push Button.= Push buttons, or pushes, are made in a variety of forms, with metal, wood, hard rubber, or porcelain bases.
Fig. 16 has a metal base, and is suitable for a front door.
Fig. 17 is a wooden pear push, and is attached at the end of a cord which has the two conductors braided in it, each, however, having its own insulation.
Fig. 18 is a plate push for an outside door.
Fig. 19 is either of metal, wood, or porcelain, and is the shape most commonly used.
A three-point push has three contact springs. One is movable by means of the button, one is below the movable spring, and the third is above it.
When the push button is not being depressed, the movable spring makes contact with the upper spring. But when the button is depressed, these two springs part, and the movable spring makes contact with the lower one.
This style of push is used for special bell and annunciator work, as will be described later.
The form of combination floor and table push in Fig. 20 is the most solidly constructed device of its kind. The lower part is set in a hole bored in the flooring, the metal flange keeping it in place and preventing its slipping through.
The floor push attachment works as follows: The central metal rod is divided into two parts _B D_, by an insulating piece of hard rubber. When depressed against the action of the spiral spring by the foot, the upper part _B_ connects together the contact springs _A C_, closing the circuit of bell and battery. These contact springs are insulated from each other by a hard rubber block _R_.