ON THE CONSERVATION OF FORCE. 313 



engine changes chemical into mechanical force the magneto-elec- 

 trical machine transforms mechanical force into chemical. 



The application of electrical currents opens out a large 

 number of relations between the various natural forces. We 

 have decomposed water into its elements by such currents, and 

 should be able to decompose a large number of other chemical 

 compounds. On the other hand, in ordinary galvanic batteries 

 electrical currents'are produced by chemical forces. 



In all conductors through which electrical currents pass they 

 produce heat ; I stretch a thin platinum wire between the ends 

 n and p of the galvanic battery, Fig. 49 ; it becomes ignited 

 and melts. On the other hand, electrical currents are pro- 

 duced by heat in what are called thermo-electric elements. 



Iron which is brought near a spiral of copper wire, traversed 

 by an electrical current, becomes magnetic, and then attracts 

 other pieces of iron, or a suitably placed steel magnet. We thus 

 obtain mechanical actions which meet with extended applications 

 in the electrical telegraph, for instance. Fig. 51, represents a 

 Morse's telegraph in one-third of the natural size. The essen- 

 tial part is a horse-shoe shaped iron core, which stands in the 

 copper spirals b b. Just over the top of this is a small steel 

 magnet c c, which is attracted the moment an electrical current, 

 arriving by the telegraph wire, traverses the spirals b b. The 

 magnet c c is rigidly fixed in the lever d d, at the other end of 

 which is a style ; this makes a mark on a paper band, drawn 

 by a clock-work, as often and as long as c c is attracted by the 

 magnetic action of the electrical current. Conversely, by revers- 

 ing the magnetism in the iron core of the spirals bb, we should 

 obtain in them an electrical current just as we have obtained such 

 currents in the magneto-electrical machine, Fig. 50 ; in the spirals 

 of that machine there is an iron core which, by being approached 

 to the poles of the large horse-shoe magnet, is sometimes magnet- 

 ised in one and sometimes in the other direction. 



I will not accumulate examples of such relations; in subse- 

 quent lectures we shall come across them. Let us review 

 these examples once more, and recognise in them the law which 

 is common to all. 



