210 PROF. O. REYNOLDS ON THE EFFECT OF 



induced electricity in the top once round every revolution. 

 And if the top were spinning from west to east by south it 

 would be rendered magnetic, with the positive pole upper- 

 most — that is, the pole corresponding to the north pole in 

 the earth or the south pole of the needle. 



In order to show that such a current might be produced, 

 a glass cylinder, twelve inches long and four across, was 

 covered with strips of tinfoil, parallel to the axis, separated 

 by very small intervals. These strips were about six inches 

 long and an half inch wide, the intervals between them 

 being the two-hundredth of an inch. In one place there 

 was a wider interval; and from the strips adjacent to this, 

 wires were connected by means of a commutator with the 

 wires of a very delicate galvanometer. This cylinder was 

 mounted so that it could be turned at the rate of twelve 

 hundred revolutions in a minute, and brought near the 

 conductor of an electrical machine. This apparatus, after 

 it had been thoroughly tested, was found to give very de- 

 cided results. As much as 20° deflection was obtained in 

 the needle ; and the direction of this deflection depended 

 on the direction in which the cylinder was turned, and 

 on the nature of the charge in the conductor. When 

 this charge was negative, the current was in the opposite 

 direction to that of the rotation. It may be objected that 

 the measurement was not actually made on the cylinder. 

 It must, however, be remembered that it was made in the 

 circuit of metal round the cylinder, and that my object was 

 to find the relative motion of the cylinder and the electricity. 

 Altogether I think it may be taken as experimental proof of 

 the fact previously stated, that, if a steel top were spinning 

 under the inductive influence of a body charged with nega- 

 tive electricity, the effect would be that of a current round 

 the top such as would render it magnetic. 



The origin of terrestrial magnetism has not been the 

 subject of so much speculation as we might have supposed 



