340 A CENTURY OF SCIENCE 



A few years later Ampere showed how to express 

 quantitatively the force between current elements, and 

 indeed developed to a considerable degree the equiva- 

 lence between a closed circuit carrying a current and a 

 magnetic shell. So convincing was his analysis and so 

 thorough his discussion of the subject, that Maxwell said 

 of this memoir half a century later, ''The whole, theory 

 and experiment, seems as if it had leaped, full grown and 

 full armed, from the brain of the 'Newton of electricity.' 

 It is perfect in form and unassailable in accuracy; and 

 it is summed up in a formula from which all the phe- 

 nomena may be deduced, and which must always remain 

 the cardinal formula of electrodynamics." 



Shortly afterwards the dependence of a current on the 

 conductivity of the wire used and the grouping of cells 

 employed, was made clear by the work of Ohm. Many 

 of his results were obtained independently by Joseph 

 Henry (19, 400, 1831) of the Albany Academy, who 

 described in 1831 a powerful electromagnet in which a 

 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 

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



