9& DYftAMO-ELECfklC MACHINE^. 



Although machines of the continuous current class may be con- 

 structed of very large dimensions, machines of moderate size have, up 

 to the present, given the most satisfactory results. Both Gramme 

 and Siemens five arc lamp machines have given the most light per H. 

 P. of any other sizes, for feeding more than one lamp. This may be 

 owing to their having given more attention to the working out the 

 various details in those sizes than in the others. Both makers have 

 constructed machines of very much larger dimensions than those 

 named, which have given excellent results. One type of machine 

 that Gramme has built seems almost unlimited in the dimensions to 

 which it might be carried. He constructed one with two commuta- 

 tors, one on each side of the armature ring. The coils wound upon 

 the rings were joined to each commutator alternately. One to a sec- 

 tion at the right side and the next to a section at the left, and so on 

 alternately, until the whole was joined. He thus obtained as it were, 

 two distinct machines in one. The currents collected by the brushes 

 could be joined in intensity, or quantity, as might be desired. To 

 further increase the power, he constructed four magnets of two coils 

 each, around the machine, the outside of which formed a poly- 

 gon, and it had two sets, or four brush collectors to each 

 commutator. By this arrangement the power of the machine was 

 again nearly doubled. The result of such combinations was very 

 nearly to quadruple the effect that he could have obtained from one 

 machine of slightly less dimensions. 



The magnetic field, as the space in which the armature revolves 

 is called, is dependent largely upon the arrangement of the electro- 

 magnets to each other, and upon the manner of winding. The mag- 

 netic effect produced by a current passing through a coil is, firstly, in 

 the ratio of the length of the conductor, or the number 

 of turns, and secondly, in the ratio of the square of the current. 

 If the number of turns of wire of which the coil of an electro-magnet 

 is composed be increased, the magnetic effect is increased in a like 

 ratio. That is to say, if the number of turns be doubled the magnetic 

 effect is doubled also. Again if the strength of the current is in- 

 creased, the magnetic effect is increased in the ratio of the square of 

 such increase up to the point of magnetic saturation. That is to say, 

 that if the current is doubled, the magnetic effect is quadrupled. It 

 has been found also, that the magnetism of an electro-magnet, for a 



