43 



ON THE ELECTROMAGNETIC EFFECT OF CONVECTION- 

 CURRENTS 



BY HENRY A. ROWLAND AND CABY T. HUTCHINSOX 

 [Philosophical Magazine [5], XXVII, 445-460, 1889] 



The first to mention the probable existence of an effect of this kind 

 was Faraday/ who says : " If a ball be electrified positively in the 

 middle of a room and then be moved in any direction, effects will be 

 produced as if a current in the same direction had existed." He was 

 led to this conclusion by reasoning from the lines of force. 



Maxwell, writing presumably in 1872 or 1873, outlines an experi- 

 ment, similar to the one now used, for the proof of this effect. 



The possibility of the magnetic action of convection-currents occurred 

 to Professor Rowland in 1868, and is recorded in a note-book of that 

 date. 



In his first experiments, made in Berlin in 1876, Prof. Rowland used 

 a horizontal hard rubber disk, coated on both sides with gold, and 

 revolving between two glass condenser-plates. Each coating of the 

 disk formed a condenser with the side of the glass nearer it; the two 

 sides of the disk were charged to the same potential. The needle was 

 placed perpendicular to a radius, above the upper condenser-plate, and 

 nearly over the edge of the disk. The diameter of the hard rubber 

 disk was 21 cm., and the speed 61 per second. 



The needle system was entirely protected from direct electrostatic 

 effect. On reversing the electrification, deflexions of from 5 to 7-5 

 mm. were obtained, after all precautions had been taken to guard 

 against possible errors. Measurements were made, and the deflexions 

 as calculated and observed agreed quite well; but it was not possible to 

 make the measurements with as great accuracy as was desired, and 

 hence the present experiment. 



Helmholtz, 2 in 1875 and later, carried out some experiments bearing 



i Experimental Researches, vol. i, art. 1644. *Wiss. Abh. i, p. 778. 



