59 o THE POPULAR SCIENCE MONTHLY. 



the resistance of the coils thus embraced by the friction-disk has to 

 be subtracted from the resistance of the circuit. 



The possibility of augmenting the strength of the current by in- 

 creasing the dimensions of the machine is too obvious to need more 

 than a passing allusion. The effects may also be increased by con- 

 necting together several such machines, as galvanic piles are con- 

 nected, either for intensity or quantity. The quality of the current 

 likewise differs according to the kind of wire surrounding the arma- 

 ture, a short thick wire producing effects of quantity, and a long thin 

 wire, of intensity. It is also easy to see that two horseshoe 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-disk between each pair of 

 poles. Moreover, the permanent horseshoe magnet may be replaced 

 by electro-magnets, which can be excited by a portion of the current 

 derived from the machine itself, according 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 cur- 

 rent to excite the electro-magnets, and the machine rapidly attains the 

 maximum effect. From conducting wires attached to the other two 

 friction-pieces a powerful current is available. 



A machine of this kind, containing two horseshoe 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 9-10ths 

 m.m. in diameter. 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 in- 

 creases, up to a maximum of TOO or 800 turns a minute, a velocity 

 which is easily obtained when steam is employed. 



Such a machine, giving an absolute continuous current of electric 

 force by the mere turning of a wheel, is of value outside the physical 

 laboratory. It is available (1) for medical purposes ; (2) for teleg- 

 raphy ; (3) for electro-plating, gilding, etc. ; (4) for military purposes, 

 signalling, explosions, etc. ; (5) for chemical decompositions ; and (6) 

 for electric illumination. 



A large machine, which has lately been exhibited in London, driven 

 by a 2^-horse-power engine, produced a light equal to 8,000 candles ; 

 a copper wire about \\ m.m. in thickness, suspended between the 

 poles, became instantly red-hot with a revolution of little over 300 in 

 a minute. Larger machines are being made that will probably give a 

 light equal to 25,000 candles. 



This machine has lately been examined by the French Soci'eth 

 cV * Encouragement, and, in accordance with the recommendation of the 



