L. Page — A Century's Progress in Physics. 307 



jar. Finally Oersted of Copenhagen undertook syste- 

 matic investigation of the effect of electricity on the mag- 

 netic needle. His researches were without result until 

 during the course of a series of lectures on "Electricity, 

 Galvanism, and Magnetism" delivered during the winter 

 of 1819-20 it occurred to him to investigate the action of 

 an electric current on a magnetic needle. At first he 

 placed the wire bearing the current at right angles to the 

 needle, with, of course, no result ; then it occurred to 

 him to place it parallel. A deflection was observed, for 

 to his surprise the needle insisted on turning until per- 

 pendicular to the wire. 



Oersted's discovery that an electric current exerts a 

 couple on a magnetic needle was followed a few months 

 later by Ampere's demonstration before the French 

 Academy that two currents flowing in the same direction 

 attract each other, while two in opposite directions repel. 

 The story goes that a critic attempted to belittle this dis- 

 covery by remarking that as it was known that two cur- 

 rents act on one and the same magnet, it was obvious 

 that they would act upon each other. Whereupon Arago 

 arose to defend his friend. Drawing two keys out of 

 his pocket he said, "Each of these keys attracts a mag- 

 net; do you believe that they therefore attract each 

 other?" 



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 bv Joseph 

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

 described in 1831 a powerful electromagnet in which a 



