If motors are wanted during the day, so that the load on the engine is nearly constant, batteries are not so valuable, except with a view of preventing a breakdown; but, if batteries cannot be used, the advantage of using motors becomes enormous, as the plant has to be large enough to supply the maximum load, and would otherwise be idle during the day. In alternating current systems there are two difficulties in the way of using motors. It is difficult to make an alternating current motor that will start, and, if that difficulty is surmounted, it is difficult to make an alternating current motor that will work on varying loads without great waste of power. The question of the efficiency of alternating current motors has never been really practically studied yet; and, until these difficulties are overcome, we must regard alternating current motors as non-existent. Several methods of working alternating current transformers off direct currents by commutating the primary have been proposed at different times; but they all seem to be impracticable, and it seems impossible to get over the difficulties that arise from sparking when it is attempted to break a high-tension circuit. One of the first methods of distribution over large areas proposed was by means of motors and dynamos combined. For instance, suppose, in order to keep down the size of the leads, 2,000 volts are used in the mains, a motor capable of working with 2,000 volts is put down where the lights are wanted, and this is made to drive a dynamo giving 100 volts and a large current. Instead of having a separate motor and a dynamo connected by a belt or by coupling the spindles together, it is simpler to make one machine with two armatures, or to have only one armature with two circuits on it. One circuit is wound with fine wire and takes the 2,000 volts and tends to turn the armature round. The other circuit is wound with thick wire giving 100 volts and a large current, and tends to stop the armature, thus absorbing the power supplied by the high-pressure circuit. The direct-current transformer or dynamotor is thus a sort of double dynamo, or dynamo and motor combined. If it gets 2,000 volts and 10 ampères, it would, if there were no waste, give 100 volts and 200 ampères; with a waste of 10 per cent., it will give 100 volts and 180 ampères.