308 L. Page — A Century's Progress in Physics. 



great many coils of wire insulated with silk were wound 

 around an iron core and connected in parallel with a sin- 

 gle cell. He remarks in this paper that with long wires, 

 as in the telegraph, many cells arranged in series should 

 be used, whereas for several short wires connected in 

 parallel a single cell with large plates is more efficient. 



Current Induction. — Impressed by the fact that elec- 

 tric charges have the power of inducing other charges 

 on neighboring conductors without coming into contact 

 with them, Faraday was engaged in investigating the 

 possibility of an analogous phenomenon in the case of 

 electric currents. His idea at first seems to have been 

 that a current should induce another current in any 

 closed conducting circuit which happens to be in its 

 vicinity. Experiment readily showed the falsity of this 

 conception, but a brief deflection of the galvanometer in 

 the secondary circuit was noticed at the instant of mak- 

 ing and breaking the current in the primary. Further 

 experiments showed that thrusting a permanent steel 

 magnet into a coil connected to a galvanometer caused 

 the needle to deflect. In fact Faraday's report to the 

 Eoyal Society on November 24th, 1831, contains a com- 

 plete account of all experimental methods available for 

 inducing a current in a closed circuit. 



While Faraday is entitled to credit for the discovery of 

 current induction by virtue of the priority of his publica- 

 tion, it must not pass unnoticed that Henry obtained 

 many of the same experimental results independently 

 and some even earlier. Henry was at this time instruc- 

 tor in mathematics at the Albany Academy, and seven 

 hours of teaching a day made it well-nigh impossible to 

 carry on original research except during the vacation 

 month of August. As early as the summer of 1830 he 

 had wound 30 feet of copper wire around the armature 

 of a horseshoe electromagnet and connected it to a gal- 

 vanometer. When the magnet was excited, a momen- 

 tary deflection was observed. "I was, however, much 

 surprised," he says, "to see the needle suddenly 

 deflected from a state of rest to about 20° to the east, or 

 in a contrary direction, when the battery was withdrawn 

 from the acid, and again deflected to the west when 

 it was re-immersed.' ' In addition a deflection was 

 obtained by detaching the armature from the magnet, 

 or by bringing it again into contact. Had the results of 



