Before going on to discuss the 'accumulator' or 'storage battery' system of electric traction, reference should be made to an invention which holds the germ of great economies in electric traction. This invention is known under the name of 'regenerative control.'

It has already been explained that the dynamo is reversible—that is to say, a dynamo may act as a motor, or a motor as a dynamo. This fact is usefully applied in braking tramcars. When a car has gained speed, its momentum represents a certain amount of stored energy. In stopping the car, this energy has to be absorbed or dissipated in some way or other. One method is to utilise the friction of brake blocks on the wheels, or of skids on the rails themselves. With the electric car, however, it is possible to absorb the energy by making it drive the motors as if they were dynamos. The moving car drives the wheels, which in turn drive the motors; and the current so generated may either be absorbed in electrical 'resistances' or led to electro-magnets which are so placed that they exercise a retarding pull on the rails. In any of these cases a car which is being stopped, or is being 'held back' by the brakes when going down-hill, is wasting power. It is clear, therefore, that a great deal of power could be saved if the current generated by the motors in retarding could be pumped back, as it were, into the electrical circuit.

This is the problem of 'regeneration' which has fascinated many electrical engineers. The practical difficulties underlying it are very great; and perhaps the only man to get within measurable distance of surmounting them was Mr J. S. Raworth, whose system of regenerative control was tried on a number of tramway systems and installed on the Rawstenstall tramways in 1909. It cannot be said with confidence that all the difficulties have been overcome; on the other hand, it would be rash to say that they are insurmountable. Mr Raworth, at any rate, retains his faith in ultimate victory; and the theoretical beauty of the system is so complete that it is bound to retain its fascination.

The practical result of regeneration is to eliminate the effect of hills. A regenerative car in descending a hill gives back to the generating station some of the excess energy required to take it up the hill. In the same way each car, in coming to a standstill, gives back a portion of the energy required to start it. A regenerative tramway may thus be represented, from the energy point of view, as one in which all the cars are running at normal speeds on level roads.

Incidentally the regenerative system gives a very perfect control of the speed of the car on all gradients, owing to the regeneration which begins automatically when the motors start 'coasting.' It is a power-saver and a brake in one; and its efficacy as a means of control is so great that, if its incidental drawbacks could be avoided, it would be worth adopting for this purpose alone, both on electric tramways and on electric railways.