The terminal L is connected to the other terminal of the lamp.
The shunt contains a small electro-magnet E mounted upon a square wooden slab or baseboard with its armature a, a contact c, and, below the slab, a resistance coil W, which is equal to the resistance of the electric arc of the light, about 1 S. u.[W]
As long as the lamp is burning well, the current circulates in the coils of the electro-magnet, and the armature a being strongly attracted, there is no contact at c. The resistance coil W is therefore not in electrical circuit. When the light is extinguished the current in the coils of the electro-magnet ceases, and the armature is withdrawn by the spring f making contact at c. This offers to the electric current a path through W of equal resistance to that of the lamp, and the current is subjected to scarcely any change, so that the motor has practically no cause to alter its rate.
When the carbon points of the lamp again touch, the electric current returns to them, breaking contact at c, re-establishing the former conditions.
Direction of Rotation.—The armature may revolve in either direction. If it becomes necessary to drive it in the opposite direction to that for which the machine has been made, it is only necessary to reverse the brushes, placing their points in the direction of motion, and to change two of the wire connections, which operations can be effected in a few minutes. [Fig. 166] shows the position of brushes for one direction of rotation and [Fig. 168] that for the other.
Conducting or Leading Wires.—The leading wires are usually of copper of high electrical conductivity. They must be insulated from one another the whole of their length and not placed too close together. As their resistance affects the intensity of the light very much, the section must be carefully proportioned to the distance of the lamp from the machine.
The best practical result is obtained when their resistance together with that of the lamp is equal to the total internal resistance of the dynamo machine. Wires of various sizes are therefore required.
Decrease in strength of the current caused by a leading wire of too high resistance can be overcome by a higher velocity, which is obtained only by increased motive power, but if the wire is much too small, it will become heated. The proper remedy is to increase the sectional area of the leading wire.