(") 47 



Again, if electricity be our starting point, we may ac- 

 complish its conversion into the other forces. Heat 

 results whenever its passage is interrupted or resisted ; 

 a wire of the poorly conducting metal platinum becom- 

 ing even red hot by the converted electricity. To pro- 

 duce light, of course, we need only to intensify this 

 action ; the brightest artificial light known, results from 

 a direct conversion of electricity. 



Enough has now been said to establish our point. 

 What is to be particularly observed of these pieces 

 of apparatus is that they are machines especially de- 

 signed for th6 conversion of some one force into an- 

 other. And we expect of them only that conversion. 

 We pass on to consider for a moment the quantita- 

 tive relations of this mutual convertibility. We no- 

 tice, in the first place, that in all cases save one, the 

 conversion is not perfect, a part of the force used not 

 being utilized, on the one hand, and on the other, 

 other forces making their appearance simultaneously. 

 While, for example, the conversion of motion into heat 

 is quite complete, the inverse conversion is not at all so. 

 And on the other hand, when motion is converted into 

 electricity, a part of it appears as heat. This simulta- 

 neous production of many forces is well illustrated by 

 our little bell-engine, which converts the electricity of 

 the thermo-battery into magnetism, and this into motion, 

 a part of which expends itself as sound. For these 

 reasons the question " How much ?" is one not easily 

 answered in all cases. The best known of these rela- 

 tions is that between motion and heat, which was first 

 established by Mr. Joule in 1849, after seven years of 

 patient investigation. 10 The apparatus which he used is 



