120 Whitehead — Magnetic Effect of Electric Displacement. 



cases, they were each split down an element of the cylinder ; 

 these splits were covered and were on the front of the case, 

 i. e., diametral planes through them cut the beam, in its central 

 position, at right angles. The conditions were not improved 

 by the apparatus in this form ; if anything, the distm-bance 

 due to the magnetic field alone was worse, and it always 

 caused a movement of the dielectric towards the side of the 

 casing to which it happened to be nearer. Still thinking it a 

 magnetic effect due to the more intense field near the coils, a 

 single large coil which would completely surround the entire 

 case and have its center in the line of the fiber, was wound. 

 The field due to this coil would be uniform around any circle 

 concentric with the coil ; the coil was 7' 62^" high, had an 

 internal diameter of 26'6^°' and contained 360 turns of '^o. 15 

 wire. The conditions were not improved by the use of this 

 coil, and it was still impossible to look for the true effect. 

 This seemed to indicate that the disturbance was not magnetic, 

 nor could it be due to electrostatic influence of the coil since 

 the shielding was practically perfect. It was thought then 

 that the electromotive force induced in the cylindrical metal 

 case must set up a field in the slit down its side sufficient to 

 cause the disturbances. To test this the slits were closed with 

 solder, and new ones cut directly back of each of the outer 

 electrodes ; the disturbance disappeared almost entirely. Tests 

 were then made with the single large coil and also with the two 

 single coils ; the constants for the large coil introduced no con- 

 siderable difference in the calculated effect; the conditions in 

 each of these series of tests were quite good ; the electric field 

 alone caused a good steady zero position ; on closing the switch 

 of the magnetic field there was usually no deflection at all, 

 only a slight oscillation ; sometimes there was a deflection of 

 1™™ or 2™°", which, however, did not reverse on reversal of field 

 and was probably a smwival of the old disturbance. 



Fourth Form of Apparatus. — As has been pointed out, the 

 design of the electrodes in the experiments above described 

 caused a position of stable equilibrium for the dielectrics, and 

 it was thought that if the electric as well as the magnetic field 

 were uniform and constant for all positions of the beam, the 

 conditions would be greatly improved and practically as favor- 

 able as the method permits. To this end the final form of the 

 apparatus was constructed : The large coil described above was 

 used for the magnetic field ; as shown in fig. 4, the electrodes 

 were two complete rings of brass, set concentric with each 

 other and with the coil. The surfaces were carefully turned 

 and polished, and each ring was split in one place to prevent its 

 becoming a short circuited secondary circuit. The true cir- 

 cular form of the ring was maintained by bridging the split 



