312 Magneto-Electric Illumination. [July* 



number of coils encircling the iron ring, but the relation 

 between these two quantities has not yet been determined. 

 The theoretical resistance of the machine should be one- 

 fourth of the whole resistance of the wire wound round the 

 ring armature ; but the actual resistance is not so great, 

 since each friction -disc always touches several radii, R, and 

 the resistance of the coils thus embraced by the friction- 

 disc has to be subtracted from the resistance of the 

 circuit. 



The possibility of augmenting the strength of the current 

 by increasing the dimensions of the machine is too obvious 

 to need more than a passing allusion. The effects may 

 also be increased by connecting together" several such 

 machines, as galvanic piles are connected, either for intensity 

 or quantity. The quality of the current likewise differs 

 according to the kind of wire surrounding the armature, a 

 short thick wire producing effects of quantity, and a long 

 thin wire, of intensity. It is also easy to see that two horse- 

 shoe magnets, instead of one, may be made to act on one 

 ring armature ; that is to say, it may be actuated by 

 four poles instead of two, or even by a greater number ; 

 always having a friction-disc between each pair of poles. 

 Moreover, the permanent horse-shoe magnet may be re- 

 placed by electro-magnets, which can be excited by a por- 

 tion of the current derived from the machine itself, accord- 

 ing to the now well-known method. At the beginning of 

 rotation the residual magnetism of these electro-magnets 

 will induce a feeble current in the ring ; one half of this 

 passes round the electro-magnets, the four poles of which 

 react on the armature. Of the four friction pieces, two 

 carry half the current to excite the electro-magnets, and the 

 machine rapidly attains the maximum effect. From con- 

 ducting wires attached to the other two friction pieces a 

 powerful current is available. 



A machine of this kind, containing two horse-shoe electro- 

 magnets, one for exciting and the other for the exterior 

 current, and having round each pole 7 kilos, of copper wire 

 3 m.m. diameter, when worked by hand, decomposes water, 

 and fuses 26 centims. of iron wire g-ioths m.m. in dia- 

 meter. However slowly the armature is rotated, the needle 

 of a large galvanometer having the wire only once round is 

 deflected, and the effects increase in intensity as the 

 velocity of rotation increases, up to a maximum of 700 or 

 800 turns a minute, a velocity which is easily obtained when 

 steam is employed. 



Such a machine, giving an absolutely continuous current 



