CHAP, vii.] MAGNETO-ELECTRIC MACHINES. 



669 



The following is a description of Messrs. Siemens' electric light 

 apparatus. It is a complete apparatus by itself, in which the core of 

 the armature is fixed, and the wire-helix alone caused to rotate. By 

 fixation of the armature core great inductive power is obtained, and 

 consequently powerful currents. With about 380 revolutions of the 

 wire-helix per- minute, and nine to ten horse-power, a light equal to 

 14,000 candles is obtained. 



In this machine (shown in Figs. 439 and 440) the conductor, by the 

 motion of which the electrical current is produced, is of insulated 



FIG. 438. Gramme machine for electric light (latest form). 



copper wire, coiled in several lengths, and with many convolutions on 

 a cylinder of thin German silver, and in such a manner that each 

 single convolution describes the longitudinal section of the cylinder. 

 The whole surface of the metal cylinder is thus covered with wire, 

 forming a second cylinder closed on all sides (, Z>, c, d, Fig. 440). 



This hollow cylinder of wire incloses the stationary core of soft 

 iron (n s s ri Fig. 440) which is fixed by means of an iron bar in the 

 direction of its axis, prolonged at both ends through the bearings of 



