478 MEMORIAL OF JOSEPH HENRY. 



deflected toward your left hand. In the same paper, he says that 

 he will soon experiment with spirals and helices of wire which, he 

 predicts, will have the same properties as magnets as long as a cur- 

 rent of electricity flows through them. He then gives his well- 

 known hypothesis of the nature of a magnet. He says that if 

 we assume a magnet to consist of an assemblage of minute currents 

 of electricity whirling all with the same direction of rotation around 

 the steel molecules and in planes at right angles to the axis of the 

 bar, we will have an hypothesis which will account for all the 

 known properties of a magnet. Ampere constructed his spirals 

 and helices, and to the astonishment of the scientific world made 

 magnets formed only of spools of copper wire traversed by electric 

 currents. We can readily imagine the intense interest awakened 

 by this discovery; a discovery which caused Arago to exclaim: 

 "What would Newton, Halley, Dufay, JEpinus, Franklin, and 

 Coulomb have said if one had told them that the day would come 

 when a navigator would be able to lay the course of his vessel 

 without a magnetic needle and solely by means of electric currents?" 



" For several weeks physicists of France and from abroad crowded 

 Ampere's humble study in Rue Foss6e Saint Victor, to see with aston- 

 ishment a suspended loop of wire, in the circuit of a battery, take a 

 definite position through the directive magnetic action of the earth." 



This hypothesis of Ampere had a powerful hold on Henry's mind, 

 and as I know that he used it as a guiding light in his researches, it 

 may here be well to give Arago's account of how Ampere was led to 

 its conception : 



" Thanks to the profound researches of Ampere, the law which 

 governs celestial movements, the law, extended by Coulomb to the 

 phenomena of electricity at rest or in tension, and then, though with 

 less certainty, to magnetic phenomena, becomes one of the character- 

 istic features of the powers exercised by electricity in motion. The 

 general formula which gives the value of the mutual actions of the 

 infinitely small elements of currents once understood, the determi- 

 nation of the combined actions of limited currents of different forms 

 becomes a simple problem of integral analysis. Ampere did not fail 

 to follow out these applications of his discoveries. He first tried to 

 discover how a rectilinear current acts on a system of circular closed 



