308 Magneto-Electric Illumination. [July, 



however, it was thought that these difficulties were inherent 

 to the magneto-electric machine, until electricians found, 

 first, that the almost instantaneous flash of the current 

 could be .considerably lengthened out, and then that the 

 successive waves generated could be so produced as to 

 flow in the same instead of in opposite directions. 



These important desiderata are supplied in a magneto- 

 electric machine of a novel form, invented by M. Gramme. 

 The principle is not difficult to understand. Take a long 

 bar of soft iron, e, e', Fig. I, round which is coiled an insu- 

 lated copper wire ; to this bar, forming an electro-magnet, 

 let a permanent magnet, s n, be presented, the south pole 

 being nearest to the iron bar. Now move the permanent 

 magnet in the direction of the arrow parallel with itself, 

 with a uniform velocity, and always maintaining the same 

 distance from the bar. The south pole of the permanent 

 magnet will produce a north magnetic pole in the portion of 

 the iron bar nearest to it ; and the gradual displacement of 

 this pole from one end to the other of the iron bar, caused 

 by the motion of the magnet, will induce in the surrounding 

 wire an electric current which may be rendered evident by 

 the galvanometer, g. This current will not be instan- 

 taneous : it will continue to flow during the whole time the 

 magnet is moving between the two ends, E e', of the iron 

 bar, and its time of duration may therefore be varied at 

 pleasure. 



This experiment shows that it may be possible, by proper 

 arrangements, to realise a machine which will furnish a 

 continuous current of electricity for as long as may be 

 desired. We have only to imagine the electro-magnet, 

 instead of being the straight bar shown in Fig. I, bent 

 into a circular form as at e, e', e", e"', Fig. 2. 



Submit this annular electro-magnet simultaneously to the 

 influence of the two poles of the permanent horse-shoe 

 magnet, N s, and at the same time imagine it to revolve 

 on its axis in the direction shown by the arrows. 



The south pole, s, of the horse-shoe magnet will produce 

 in that portion of the ring, e, which is near it an electric 

 current in a particular direction, as may be inferred from 

 what we have said respecting the straight bar, Fig. i. But 

 the north pole, N, of the magnet will likewise produce in 

 the part of the ring which is in its neighbourhood, e", an 

 electric current flowing in the opposite direction ; and it is 

 easily conceived that in the two portions of the ring, e' and 

 e'", which are in what may be called the mean position, 

 there is no current at all. If, therefore, we wish to collect 



