The intensity of the unassisted light is given in standard candles. The standard here used is a stearine candle consuming 10 grammes per hour.
Regulation.—From the fact that a closed circuit rotating in a magnetic field experiences resistance to its motion which a broken circuit does not, motive power to any extent is only required when the circuit is closed. An interruption of the current is therefore equivalent to removing the load from the motor, which for mechanical reasons may be injurious to it and for electrical reasons to the dynamo machine.
The sudden interruption of the circuit of the large machine produces an electric tension so dangerously high as to strain or destroy the insulation of the machine. When contact is again made after such interruption, the increase of speed resulting from the interruption causes a momentary current of great intensity, accompanied by sparks at the commutator.
In order that the light may be quite steady the speed should be as uniform as possible. As too high an increase of speed may result in temporary extinction of the light, it ought never to be permitted. The motor should therefore be provided with a good and sensitive governor, that will keep the speed perfectly uniform however the steam and load may vary. A large and heavy fly-wheel is also very useful in keeping the speed nearly uniform during change of load.
Although the circuit, when the machine is in full action, should never be suddenly interrupted, interruption arising from the extinction of the light is not dangerous, because it is always preceded by a decrease in the strength of the current. When it is desired to divert the current into another circuit it is advisable to stop the machine. Although in practice with small machines this is rarely done, with large machines it is necessary.
Self-acting Shunt.—For great security, especially with the two machines coupled together, where the electric current is strong and the light equivalent to about 14,000 candles, it is advisable to insert in the circuit a self-acting shunt.
This is placed between the lamp and machine and connected to both leading wires. Its principle is as follows:—
The terminal M, [Fig. 167], is joined by a short connecting wire to one terminal of the machine. The terminal L M is connected to the remaining terminal of the machine and also to one of the lamp terminals.