A CENTURY'S PEOGEESS IN PHYSICS 351 



rings of eight spheres each so that they could be rotated 

 about their common axis from fifty to sixty times a sec- 

 ond. One set of spheres was connected by brushes to the 

 positive pole of a battery of 20,000 volts, the other to the 

 negative pole. The deflection of a nearby magnetometer 

 needle was observed when the electrification of the two 

 rings was reversed, and from the reading so obtained the 

 ratio of the electromagnetic to the electrostatic unit of 

 current computed. This quantity was found to differ 

 from the velocity of light by only a few percent. This 

 experiment and the even more exhaustive investigations 

 carried out by Pender, both independently and in collab- 

 oration with Cremieu, finally convinced the scientific 

 world that a convection current produces the same mag- 

 netic field as a conduction current of the same magnitude. 



In discussing the ponderomotive force experienced in a 

 magnetic field by a conductor through which a current is 

 passing, Maxwell had said, "It must be carefully remem- 

 bered, that the mechanical force which urges a conductor 

 carrying a current across the lines of magnetic force, 

 acts, not on the electric current, but on the conductor 

 which carries it." Hall (19, 200, 1880), one of Eow- 

 land's students, questioned this statement, and deter- 

 mined to put it to the test of experiment. Efforts to find 

 an increase in the resistance of a wire placed at right 

 angles to the lines of magnetic force were unsuccessful. 

 So the current was passed through a moderately broad 

 strip of gold leaf and the effect of the magnetic field 

 on the equipotential lines investigated. The results 

 obtained confirmed Hall's belief that the force exerted by 

 the field acts on the current itself, and is transmitted 

 through it to the conductor. Further investigation (20, 

 161, 1880) revealed the same deflection of equipotential 

 lines in thin strips of other metals, although the effect 

 was found to be reversed in iron. 



During the closing years of the nineteenth century 

 occurred three events of far reaching importance. The 

 electron was isolated, and its charge and mass measured 

 by J. J. Thomson in England; X-rays were discovered 

 by Eontgen in Germany; and the first indications of 

 radioactivity were found by Becquerel in France. The 

 first two are certainly to be attributed largely to the 



