Page:Motors and motor-driving (1902).djvu/339

Rh common proceeding. For this purpose the car should be provided with a plug and short length of cable, which should be carried with it. It is also advisable to have variable resistance. In charging the battery the cells are of course all arranged in series. Forty-four cells are often selected as the number employed, as such a battery can be conveniently charged at any ordinary direct current electric light station, which are usually designed to supply current at 110 volts or more. It is as well that every car should be provided with a switch which can be used for interrupting all connection between the cells and the motors of the vehicle while charging, as otherwise somebody moving the controller may cause the car to start off suddenly at full speed and take a header into a wall or a piece of moving machinery. A lock for the controller is nearly as good.

It must be borne in mind in charging a battery of accumulators that the voltage required is in excess of that which the cells give when discharging. Thus a forty-four cell battery will give about eighty-eight to ninety volts for car propulsion, but it will require something over one hundred volts to charge it. The variable resistance mentioned above should be inserted in the circuit so as to enable the amount of current charged into the cells to be controlled. When it is desired to economise the time occupied in charging, more current may be run in during the earlier stages, the amount being gradually diminished towards the end of the charge. The approach of the completion of the charge is marked by the 'gasing' of the cells—that is, the dilute acid fizzes more or less like soda water. The strength of the acid also increases as the cells are charged, and the increase may be used to show when charging is complete. A small glass vessel called a hydrometer, which can be inserted into the acid, and indicates its specific gravity, is used for this purpose.

Of course batteries can be charged from any electric lighting circuit that gives the necessary pressure or a higher one, as the forty-four-cell battery could be charged from a