ON THE CONSERVATION OP FORCE 



217 



in them an electrical current just as we have obtained such 

 currents in the magneto-electrical machine, FIG. 102; in the 



FIG. 103 



spirals of that machine there is an iron core which, by 

 being approached to the poles of the large horse-shoe mag- 

 net, is sometimes magnetised in one and sometimes in the 

 other direction. 



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

 sequent lectures we shall come across them. Let us review 

 these examples once more, and recognise in them the law 

 which is common to all. 



A raised weight can produce work, but in doing so it must 

 necessarily sink from its height, and. when it has fallen as 

 deep as it can fall, its gravity remains as before, but it 

 can no longer do work. 



A stretched spring can do work, but in so doing it becomes 

 loose. The velocity of a moving mass can do work, but in 

 doing so it comes to rest. Heat can perform work; it is 

 destroyed in the operation. Chemical forces can perform 

 work, but they exhaust themselves in the effort. 



Electrical currents can perform work, but to keep them up 



