produced hy varying Magnetic Induction. 477 



A distinct and proper deflexion was now observed. 



A wire was then taken 10 times round the ring, and con- 

 nected to a common quadrant electrometer. The deflexion 

 was easy to see. It could also be just seen with only one turn 

 of wire. 



This kind of experiment seems likely to furnish a method 

 of determining "u^^; and it ought to be a good method, because 

 it is v^ which is really measured. 



However, what I had wanted to see was not any effect 

 assisted by the concentration of a conductor, but a result in free 

 dielectric. And this at length has been obtained. 



All copper screens and everything of the sort were removed. 

 The ring was wound with only a few turns of thick wire so 

 as to reduce the slope of potential needed to propel a cur- 

 rent through it. The wire was wound symmetrically in two 

 halves, and the far ends were connected to a plug-key, so that 

 one might work with either closed or open circuit. 



The reversing-key was a quite distinct mercury arrange- 

 ment specially made for strong currents, fixed to a table on 

 which stood the screen for receiving the spot of light, which 

 was a small disk of translucent paper in a drum of tin plate 

 mounted on an independent stand. The light of a lamp was 

 brought on to the mirror independently of this stand by a 45° 

 looking-glass. Such a screen is handy,because one can follow 

 the rather erratic spot of light all over the table, wherever it 

 chooses to take up its temporary' abode. 



The middle of the battery was put to earth, so that the 

 potentials of the two halves of the wire might be equal and 

 opposite. The needle was then adjusted by trial and error, 

 its glass box being pushed in and out of the ring, its sus- 

 pending fibre being turned by a tangent screw, &c., until the 

 electrostatic effect obtained by working the reverser on open 

 circuit was very small. It could never be got quite zero ; 

 and even when small it would not always remain small for 

 long. 



However, on now closing the circuit by the plug-key, the 

 residual electrostatic effect would be certainly largely reduced, 

 and the vvished-for effect might be seen. It was seen ; the 

 hair-line of the spot of light oscillated over two or three 

 millimetres when the key was worked in unison with the 

 %'ibrating arm. It assisted the residue of the electrostatic 

 effect when the charging of the needle was in one direction, 

 and it neutralized or reversed the residual electrostatic effect 

 when the charging of the needle was inverted. 



It is this reversal of whatever disturbing effect remained 

 Phil. Mag. S. 5. Vol. 27. No. 169. June 1889. 2 K 



