In the United States it is usual to place an alternating current motor in each house to be lighted; but the conditions are quite different there, overhead wires being used extensively. In this country this system is not likely to find favour, and local sub-stations will be used, the high pressure, which is always dangerous to life, will thus be kept out of private houses and offices. There is, then, very little difference in the cost of maintenance of alternating current transformers and dynamotors, and the advantages possessed by alternating current transformers in this respect are more than counter-balanced by the use of motors on direct current circuits. Dynamotors have not come into general use yet because no stations have been started in this country of the size which demands them. No central station with sub-stations is in operation, but there is every reason to expect several will be soon; and it is very necessary to discuss the various methods, not only in use at this moment but coming into use in the immediate future. The dynamotor itself needs no working out, as any maker of direct-current dynamos can, of course, make them. Messrs. Paris and Scott of Norwich showed some in operation at the Newcastle Exhibition in 1887; the most successful type is that recently invented by Mr. Jas. Swinburne, illustrated by Fig. 29. The backward main round primary or motor magnet is shown on the left, and the forward main round the secondary or dynamo magnet on the right, the outside coil round both magnets is the shunt. [Illustration: FIG. 29.] The dynamotor may be made with two circuits on one armature as already explained, or it may have two armatures in separate fields, still making up one machine. The first arrangement has two grave disadvantages. There is difficulty about securing perfect insulation between the two circuits, and this leads to chances of danger in the houses. A dynamotor with two circuits on one armature cannot be compounded, that is to say, it cannot be made to give constant electrical pressure on the mains if the number of lamps is varied. A Swinburne double armature machine can be compounded, not only to give constant pressure with a varying load, but to give constant electrical pressure even if both the load and the pressure on the primary circuit vary. This makes a considerable difference in the copper of the primary leads, as in large and complicated districts it is almost impossible to arrange leads, even when working with high electrical pressure and small currents, so that the electrical pressure remains constant, or even nearly so. A very small variation of the pressure on an incandescent lamp makes an enormous difference in the amount of light it gives, and in its duration. It is, therefore, most important that the E. M. F. on the lamps should be kept absolutely constant.