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use light only an hour or two, others require it for several hours.
These conditions are met by the attendant at the station who, by means of the various devices for regulating the current, keeps a pressure upon and a quantity flowing over the wires only sufficient for the number of lights in use at any given time.
As the lights are turned on or off the current increases or diminishes, and up to a certain limit this change is met by using the regulators.
When the current becomes too great to be controlled by these, one or more units are thrown out of operation; that is to say, one couple of dynamos is stopped, and then another, and so on as the lights are gradually turned off. On the other hand, as lights are turned on and the supply of current increases, one couple after another is put in operation to keep pace with the demand.
To make this clearer, we will say that two dynamos are started late in the afternoon a little before the time when
