Jtim 17, 1 875 J 



NATURE 



131 



tion, the intensity of which will increase till the loop 

 reaches s', after which the current, always preserving the 

 same direction, will diminish till the loop arrives at o', 

 when the current will for a moment fall to zero, to be 

 succeeded by a current in the opposite direction as the 

 loop leaves o'. This current will in like manner increase 

 during the advance of the loop to n', when it will attain 

 a maximum, and afterwards diminish till it arrives at o, 

 where, after passing through zero, the direction will again 

 change. There will thus be a current always flowing 

 in one direction as the loop moves from O through s' to 

 o', and in an opposite direction as it moves from o' 

 through n' to o. Now if the loop, instead of being 

 moveable upon thejring, be firmly attached to it, and the 

 ring itself carrying the loop be rotated on its axis in the 

 plane of the fixed magnet N M s, it will be tound that the 

 currents developed will correspond both in direction and 



intensity with those produced in the moveable loop, pro- 

 vided we allow for the small displacement in the position 

 of the poles of the ring arising from its motion. 



The foregoing statement may be extended from a single 

 loop to any number of loops forming part of a coil ex- 

 tending over the whole of the iron ring (Fig. 4). Each 

 loop of such a coil, during one-half of every revolution, 

 will tend to give a current in one direction, and during 

 the other half, a current in the opposite direction, and the 

 electromotive force thus produced will augment with the 

 number of loops in the coil. If, then, metallic conductors, 

 c €,</ c', are applied to the loops (whose surfaces must be 

 exposed at one point for this purpose) as they pass through 

 the positions O and o', continuous currents, all in the 

 same direction, will be obtained on rotating the ring with- 

 out the use of a commutator, unless we apply that term 

 (as Pacinotti has done) to the system of conductors or 

 rheophori by which the currents are carried off. 



In order to obtain currents of high intensity, the single 

 coil must be replaced, as in similar machines, by a num- 

 ber of coils of thin wire rolled one above the other and 

 carefully insulated. To carry oft the current, these coils 

 must be divided into separate helices, with the adjacent 

 terminals of the 'wires of the helices in metallic connec- 

 tion, so that the iron ring may be always surrounded by 

 an endless conductor of great length. I have already 

 described the arrangements adopted in the transversal 

 electro-magnet of Pacinotti. The construction of the 

 ring armature in Gramme's machine will be readily un- 

 derstood from Fig. 6, in which it is represented in diffe- 

 rent stages of its construction, so as to show the manner 

 in which the principal parts are connected.* At A a sec- 

 tion of the iron ring itself is shown, composed of a bundle 

 of iron wires ; at B B the helices, or bobbins, are seen 

 both in section and detached ; and at R r the form is 

 shown of one of the insulated copper conductors, to 

 which the contiguous ends of the wires of the helices are 

 attached, and from which the current is drawn off by 

 means of rubbers or brushes formed of flexible bundles of 

 copper wire. These brushes are so applied at the neutral 

 positions of the ring that they begin to touch one of the 

 conductors R, before they have left the preceding one. In 

 this way no actual break or interruption occurs in the 

 current. The permanent magnets employed in the smaller 



Fig. 6.— Gramme Armature. 



Gramme machines are on the improved construction of 

 M. Jamin. 



With a small machine, on the Gramme construction, 

 very remarkable electrical effects may be obtained. I 

 will give the results of a few experiments which I recently 

 made with one of the two machines exhibited at the late 

 meeting of the British Association, and which are now in 

 Queen's College, Belfast. This machine was able to heat 

 to full ignition in daylight a platinum wire one foot in length, 

 and weighing 12 grains. With a voltameter formed of two 

 slips of platinum foil, exposing each a surface of 1*25 

 square inches, and at the distance of half an inch from 

 each other, immersed in dilute sulphuric acid, water was 

 freely decomposed. For 100 turns of the machine, the 

 volumes of the mixed gases collected at different rates of 

 turning were as follows : — 



In 34 seconds 260 cubic inches 



„ 45 » 2-53 



» 75 » i'45 



„ 135 » o"35 » 



From these observations it appears that, under the con- 

 ditions of this experiment, the quantity of water decom- 

 posed for the same number of revolutions of the ring 

 increases quickly with the rate of the motion till a certain 



• I take this opportunity of expressing my obligations to M. A. Niaudet- 

 Breguet for his kindness in enabling me to give the admirable figures of the 

 Gramme Machine which illustrate this paper. They first appeared in a 

 short work on the Gramme Machine, recently published by M. Breguet, to 

 which I beg to refer for more detailed information regarding its practical 

 applications (" Machines Magneto-electriques Gramme." Par M. A. Niaudet- 

 Brcguet. Paris, 1875). 



