12 



ON THE MAGNETIC EFFECT OF ELECTEIC CONVECTION * 



[American Journal of Science 13], XV, 30-38, 1878] 



The experiments described in this paper were made with a view of 

 determining whether or not an electrified body in motion produces 

 magnetic effects. There seems to be no theoretical ground upon which 

 we can settle the question, seeing that the magnetic action of a con- 

 ducted electric current may be ascribed to some mutual action between 

 the conductor and the current. Hence an experiment is of value. Pro- 

 fessor Maxwell, in his ' Treatise on Electricity/ Art. 770, has computed 

 the magnetic action of a moving electrified surface, but that the action 

 exists has not yet been proved experimentally or theoretically. 



The apparatus employed consisted of a vulcanite disc 21-1 centi- 

 metres in diameter and -5 centimetre thick which could be made to 

 revolve around a vertical axis with a velocity of 61- turns per second. 

 On either side of the disc at a distance of -6 cm. were fixed glass plates 

 having a diameter of 38-9 cm. and a hole in the centre of 7-8 cm. The 

 vulcanite disc was gilded on both sides and the glass plates had an 

 annular ring of gilt on one side, the outside and inside diameters being 

 24-0 cm. and 8-9 cm. respectively. The gilt sides could be turned 

 toward or from the revolving disc but were usually turned toward it so 

 that the problem might be calculated more readily and there should 

 be no uncertainty as to the electrification. The outside plates were 

 usually connected with the earth; and the inside disc with an electric 

 battery, by means of a point which approached within one-third of a 

 millimetre of the edge and turned toward it. As the edge was broad, 

 the point would not discharge unless there was a difference of potential 

 between it and the edge. Between the electric battery and the disc, 



1 The experiments described were made in the laboratory of the Berlin University 

 through the kindness of Professor Helmholtz, to whose advice they are greatly in- 

 debted for their completeness. The idea of the experiment first occurred to me in 

 1868 and was recorded in a note book of that date. 



