Page:Motors and motor-driving (1902).djvu/319

Rh these is the Joel, in which the armature constructed on the drum principle is arranged outside an eight-pole field, round which it revolves.

The modern electric motor is the most efficient machine in existence. Motors can be constructed which convert upwards of ninety per cent, of the electric energy supplied to them into mechanical energy. The efficiency declines as the size of the motor. diminishes, but motors of an eighty per cent, efficiency are practicable for automobile purposes.

The electric motor is also exceedingly compact. As regards the amount of space it occupies, it compares favourably with any other form giving the same amount of power. As regards weight it is slightly heavier than the petrol motor.

Its leading feature, however, is its magnificent flexibility. It will start from a position of rest and run up gradually to the required speed without jolt or jar, and as varying speeds can be obtained by grouping parts in the motors and the batteries, no change-speed gears are required in an electric car, a single speed reducing transmission gear being all that is needed.

Above all, the electric motor is practically noiseless, and it emits neither visible vapour nor effluvium.

Assuming a vehicle with running gear complete and motors mounted and geared to the driving-wheels, the motors have to be fed with energy in order that the car may be propelled— that is to say, we must in addition to the motors have a source of electric current present. A car with body, running gear, and motors geared in position, is a potentiality only. It requires an electric current to vivify it and enable it to move.

Attempts have been made to propel electric vehicles with primary batteries. They have neither been successful technically nor commercially. At the present day the secondary battery or accumulator is the only adequate source of electric current for the propulsion of motor vehicles which is self-con tained and trustworthy. It is just conceivable that a car might be propelled by the recently introduced Cupron battery, but the attempt could hardly have commercial success.