takes place iu the coil of one bobbin as it revolves in the presence of the magnetic poles, P, N. If we start from the equa torial line, E E', and go by successive impulses, we End that, when the bobbin is joined with a galvanometer, the current induced is always in one direction until we come again to the equatorial line; but when we pass this, the current is reversed on the other side. This is much the same as what is found in the Siemens armature. But there is this difference here: The arinature wire with the sectors is continuous from end to end. On each side of the equatorial line we have two equal and opposite elec tric forces or batteries, and these, if left alone, would neutralize each other. But if, in the equatorial line, we introduce brushes to act as poles, we have, as it were, two gal vanic batteries joined up, as it is called, in quantity, with both positive poles together and also both negative. The brushes embrace several sectors at once, so there is nc .spark when they leave any particular sector, contact being established with the others. The conditions of the machine never alter, and hence the. current is perfectly steady, and the sectors being slways of the same sign at the points where the brushes rub, the -current is always in the same direction. Siemens and Wheatstone's principle is employed in Gramme machines. There are two fixed eleetro-magnets, and two arina tures on the same spindle; one electro-magnet and one armature being set apart for exciting both electro-rnagnets, and ale other armature and electro-magnet for sending out the external current. Astonishing as were the effects produced by Wilde's niachine, those obtained from Gramme's seem quite to eclipse them. In comparing two magneto
electric machines, we must take into account the kind of wire used for the revolving armature. For tension purposes, a thin and long wire gives the best results; for quan tity or heating purposes, a short and thick wire does best. To compare a tension with a quantity armature; the same test even in the same machine would give most contra dictory results. But comparing, so far as possible, machines intended for the same. purpose. Gramme seems to have the advantage of' all others. In the first place, the speed of revolution seldom exceeds 800 revolutions per minute; 300 is sufficient tor most purposes. A Gramme machine driven by the hand will rnelt 10 in. of an iron wire -A of an inch in diameter, a feat not accomplished by any other arrangement. The electric light got by a 3-horse-power engine working a machine a ton in weight is equal to upwards of 8,000 sperm candles. A signal-light of this kind has been con structed for the house of lords, under the superintendence of the eminent engineer Conrad. _ W. Cooke, who has rendered no small service in perfecting the machine. The carbons. consumed last for four hours, and when burned out are instantaneously replaced. A. Gramme machine adapted for electro-plating, and worked by a 1-horse-power engine, ' deposits nearly 27 oz. of silver per hour, an achievement far transcending the similar performance of other machines. Among the heating wonders of the Gramme machine we are told of a file half an inch in diameter being burnt up in 5 minutes, of 15 ft. of No. 18 platinum wire being brought to a glowing heat, and of 8 ft. of iron wire .051 inch in diameter being fused.