ELECTRICITY AND MAGNETISM 



disk at the part immediately between the magnetic 

 poles. Under these circumstances all was quiescent, 

 and the galvanometer exhibited no effect. But the 

 instant the plate moved the galvanometer was in- 

 fluenced, and by revolving the plate quickly the needle 

 could be deflected ninety degrees or more." 4 



This rotating disk was really a dynamo electric 

 machine in miniature, the first ever constructed, but 

 whose direct descendants are the ordinary dynamos. 

 Modern dynamos range in power from little machines 

 operating machinery requiring only fractions of a horse- 

 power to great dynamos operating street-car lines and 

 lighting cities ; but all are built on the same principle 

 as Faraday's rotating disk. By this discovery the use 

 of electricity as a practical and economical motive 

 power became possible. 



STORAGE BATTERIES 



When the discoveries of Faraday of electro-magnetic 

 induction had made possible the means of easily gener- 

 ating electricity, the next natural step was to find a 

 means of storing it or accumulating it. This, how- 

 ever, proved no easy matter, and as yet a practical stor- 

 age or secondary battery that is neither too cumber- 

 some, too fragile, nor too weak in its action has not been 

 invented. If a satisfactory storage battery could be 

 made, it is obvious that its revolutionary effects could 

 scarcely be overestimated. In the single field of aero- 

 nautics, it would probably solve the question of aerial 

 navigation. Little wonder, then, that inventors have 

 sought so eagerly for the invention of satisfactory stor- 



245 



