APPLICATIONS OP THE ELECTRIC LIGHT. 27 1 



" Thus there may be required in war, either a momentary illu- 

 mination, or one prolonged for perhaps the whole night. We 

 have already seen that these two illuminations can be readily 

 produced at will by the electric light, by closing or interrupting 

 the voltaic circuit." 



Martin afterwards explains the conditions for applying the 

 electric light so as to obtain these results. At the time 

 when his paper was written, however, the light could not be 

 produced by electro-magnetic machines, and it would have 

 been necessary to work with the cumbersome materials of 

 a battery, which made the problem one of much greater 

 difficulty. Now, thanks to the small dimensions of the 

 magneto-electric machines, very considerable luminous inten- 

 sities may be obtained, and this kind of application of the 

 light becomes an easy matter. The magneto-electric or, 

 best of all for this purpose, the Gramme machine, may 

 be mounted to a portable engine, which can be as easily 

 moved about to any required position as the cannons. The 

 system used in France is driven by a Brotherhood three- 

 cylinder machine. The electro- magnets of the Gramme 

 machine are thin, flat, and very wide ; the bobbin has two 

 current collectors, and a commutator mounted on the arma- 

 tures allows the machine to be joined up for tension or for 

 quantity. This plan, as shown in Fig. 71, has been adopted 

 by France, Russia, and Norway. 



According to Fontaine, it was found, by experiments made 

 at Mount Vale'rien with a machine thus arranged, that an 

 observer, placed beside the apparatus, is able to see objects 

 6,600 metres distant, and to distinguish clearly details of 

 construction at a distance of 5,200 metres. To obtain these 

 results, the Gramme machine must have a power of 2,500 

 lamps, and the projector must concentrate the light by re- 

 flection and refraction, as in those projectors we have de- 

 scribed for lighting ships at sea. 



When the electro-magnets of the machine are joined up 

 for quantity, it turns with a speed of 600 revolutions per 



