\/A- \\ii.i.L\.\i \//-;.i//-;. v.v, J-.R.S. 215 



thence outward to ignite a platinum wire of some 12" in length, or 

 rform other work. 



This machine, although the first of its kind, has done good 

 ^rvice ever since its construction, having been found very 

 rtlicaci'iiis in exciting powerful permanent magnets at the tele- 

 graph works of Siemens Brothers at Woolwich. 



Since 1807 the accumulative principle has been employed in the 

 machines of different makers, and one form of dynamo-electric 

 machine, that of M. Gramme, differs very materially from the 

 machine above referred to, and has met very deservedly with 

 extensive recognition. M. Gramme embodied in his machine the 

 principle of Professor Pacinotti's magnetic ring, which enabled 

 him to produce powerful electric currents without much of the loss 

 of energy caused in previous machines through the heating of the 

 rotating armature. 



Another modification of the dynamo-electrical machine is one 

 devised by Mr. Von Heftner Alteneck, an engineer and physicist 

 employed under my brother Werner Siemens, at Berlin. This 

 machine differs from that first submitted by myself in several 

 important particulars. Instead of the Werner Siemens armature, 

 Von Heftner Alteneck adopted a rotating coil of iron wire wound 

 with insulated copper wire in more than one direction, the several 

 coils of wire being connected seriatim with the commutator, and 

 through it, with the wire surrounding the soft iron bars, and with 

 the electric lamp or other resistance on the outer circuit. 



The advantage claimed for this mode of construction is that all 

 the wire forming the rotating coil or helix is brought into the 

 magnetic field, excepting only those portions crossing from side 

 to side of the coil; and in order to reduce this unproductive 

 resistance to a minimum, the rotating coil or helix has l>een made 

 comparatively long, and the number of electro-magnets has been 

 increased generally to six or more. 



The principal advantage of the dynamo-electrical machine over 

 all other current generators consists in its power of producing 

 currents of great magnitude, and of an intensity up to 100 volts, 

 with a small primary resistance, and therefore with a com- 

 paratively small expenditure of mechanical energy. It labours, 

 on the other hand, under the disadvantage that the power of the 

 current depends, at a given velocity, upon the magnetic force 



