CHAP. VIL] ELECTRO-MOTORS. 657 



passed to the next, an ingenious arrangement by which to lessen the 

 spark which is produced by the starting of a fresh current. The 

 oxidisation of the contacts caused by this discharge is thus greatly 

 reduced. 



III VARIOUS APPLICATIONS OF ELECTRO-MOTORS. 



Electric prime movers can never successfully compete in powet or 

 economy with those in ordinary use, such as steam-engines. None 

 have ever been constructed whose force exceeded a single horse-power. 

 The reason of this is given by the principles of the mechanical theory 

 of heat. The work of electro-motors is another form of the heat 

 which the chemical actions of the battery develope ; but since this 

 method of production of heat is much more costly than that which 

 consists in burning the coal necessary to the production of steam, 

 it necessarily follows that the motive force of electricity is much 

 less economical than that of steam. Experience has entirely con- 

 firmed this conclusion. 



But if electrical engines cannot compete, in this respect, with the 

 steam-engine and other prime movers ; ,if their employment in 

 manufactures appears impossible ; there are services of another order 

 which they can perform, whenever we require, not a particularly 

 great force, but one of great regularity and velocity, and capable 

 of acting at a great distance. Under these conditions they have 

 a superiority which is increased by the ease with which they are set 

 in action or stopped, the absence of all danger, and the small space 

 they occupy. The inventor of the rotatory engine we have just 

 described, M. Froment, made use of engines of this kind for the deli- 

 cate operations of scientific mechanics, to which he devoted himself. 

 He made use of them to move the wheels of his dividing engine, ari 

 instrument of such extreme precision that it can trace on a tube of 

 glass divisions of excessive fineness, up to 1,000 marks in one milli- 

 metre. The precision and almost infinite delicacy of this instrument 

 make it a marvel of mechanics, as may be judged by the following 

 passage in a report by M. Dumas : 



" When we were assembled in London, on the occasion of the 

 exhibition, M. Froment, in the middle of the meeting, drew out his 



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