1 6 ELEC TRIG LIGHTING. 



by grouping the elements /;/ series, that is to say, in such a 

 way as to collect for tension a greater or less number of 

 generating groups whose elements are themselves connected 

 for quantity. But farther on we shall have occasion to recur 

 to this important question ; here we shall merely add that 

 among the different generators there are some more advan- 

 tageous than others for the electric light, as regards the 

 energy of their action, as well as the expense attending 

 them, and that the proper choice of a generator is one of 

 the most important questions to be taken into consideration 

 in connection with the electric light. Thus, among batteries, 

 Grove's and Bunsen's are those which always yield the most 

 advantageous results; and among machines the magneto- 

 electric induction machines are those which must be used. 

 Nevertheless unlooked-for improvements have in quite re- 

 cent times made it possible to obtain the electric light with 

 thermo-electric piles, and we shall see that if these apparatus 

 could be made as substantial and durable as the experi- 

 ments already made seem to warrant us to expect, this system 

 would indeed be the simplest and most economical means 

 of solving the problem of electric illumination. In the 

 meantime, until proof of this has been brought forward, the 

 induction machines must still be regarded as those which 

 best answer the various requirements of the problem, and it 

 is these that are at the present time almost exclusively used. 



THE VOLTAIC ARC. 



When the electric light is formed between two conductors 

 separated by a stratum of gas, as shown in Fig. i, it receives 

 the name of the voltaic arc* and under these conditions the 



* The name of voltaic arc was given to the electric spark exchanged be- 

 tween two carbons, on account of the curvature of the luminous track when 

 the stratum of air it traverses tends to rise under .the influence of the high 

 temperature to which it is subjected. 



