OHAP. vii.] MAGNETO-ELECTRIC MACHINES. 671 



440), separated from the stationary iron, core by space only sufficient to 

 permit the free rotation of the wire cylinders. The curved bars are 

 themselves prolongations of the cores of the electro-magnets (E E E E), 

 and the sides of the two horse-shoe magnets (NO s, m and N'O' s',ra') 

 are connected by the iron of the two standards (om and o'm'). 



As the coils of the electro -magnets form a circuit with the wires of 

 the revolving cylinder, the revolution of the latter causes a powerful 

 current to pass into the electro-magnetic coils, this again inducing a 

 still more powerful current in the wires of the cylindrical armature. 

 The iron core of the cylindrical armature being very close to the poles 

 of the electro-magnets, becomes itself an intensely powerful transverse 

 magnet of opposite polarity to the electro-magnet. The cylinder of 

 wire thus revolves in a very intense magnetic field. 



These electrical currents are collected on two metal rollers or 

 brushes, so that at two points diametrically opposite the single sectors 

 pass under the rollers or brushes with elastic pressure giving up to 

 them their electrical charge. 



A slight increase of speed in the rotation of the wire cylinder is 

 followed by a considerable increase of current, but as the current 

 increases, so does the resistance to rotation ; and this very rapidly. 

 In addition to this, heat is developed to such an extent, that care must 

 be taken not to exceed a certain limit, otherwise, the insulation of the 

 coils would be destroyed. Were it not for this drawback almost any 

 amount of current might be produced with suitable driving power. 



As the external resistance affects the strength of the current the 

 speed must be varied accordingly, being greater as the external resis- 

 tance is greater, and vice versd. With an electric lamp in a circuit of 

 small resistance, if the machine is intended to work continuously, the 

 revolutions of the wire cylinder per minute should not exceed 370 to 

 380. The temperature of the machine will then be at a maximum in 

 about three hours : and during work will remain constant. At this 

 speed the driving power is about eight indicated horse-power, while 

 the intensity of the light, unaided by reflector or lens, has been shown 

 by various photometers to be equal to 14,000 normal English candles. 

 A more intense electric light cannot be obtained, as any increase in 

 the current splits up even the best carbon. 



The conducting wires from the machine to the lamp should be of 

 copper, offering very little resistance, and at the same time possessing 



