118 Whitehead — Magnetic Effect of Electric Displacement. 



tnde above indicated would to a certain extent overcome this 

 tendency. The test for the true effect seemed to lie in a 

 reversal of the sense of the deflection when the phase of either 

 the electric or the magnetic fields was changed bj 180°. 



Results. — When the electric field alone was applied, the 

 beam, as was expected, found a stable zero position near the 

 center of the electrodes. With a fiber 20^°" long this position 

 could be upset by 10 to 20°"™ on the scale, by a very small twist 

 of the torsion head. This test was not so satisfactory when the 

 long fiber was used, owing, apparently, to the large inertia of 

 the si^spended system, and to the arising of other disturbances 

 within the time necessary for the torsion of the fiber to make 

 itself felt. The zero point was always quite steady, having 

 practically no oscillation, but was not always the same, varying 

 within two centimeters. This indicated that near the center 

 there was a region of fairly uniform electric field, and also that, 

 owing to the mass and damping of the suspended system, the 

 torsion of the fiber could not be depended on to give the zero 

 position. The mass of the beam and dielectrics was from 2 to 

 3 grams, varying with the nature of the dielectric ; of this the 

 greater part was in the dielectrics. Fibers of the size here 

 required broke if the mass were 3 grams or more. 



The method of procedure was to allow the beam to come 

 to rest under the influence of the electric field alone, the long 

 time requu'ed for the oscillations about the central position to 

 die out being another evidence that the field there was fairly 

 uniform. Then the magnetic field was put on and the result- 

 ing deflection noted. Then either the electric or magnetic field 

 was reversed, the defiection noted, and so on. The deflection 

 was generally small and quite slow in all cases, requiring con- 

 siderable time to become steady ; in most of the observations 

 only the sense of the deflection was noted. Proceeding in this 

 way, several hundred observations were taken ; they were gen- 

 erally negative in result, that is, the deflection did not change 

 in either direction or amount when either field was reversed. 

 At times, however, there were indications of the eifect looked 

 for, as is shown by the following description : 



The electric field and then the magnetic field being put on, 

 as described, with the values and conditions as given above, 

 there resulted an uncertain deflection of about l"'^, which did 

 not reverse with a reversal of the magnetic field. The circuits 

 were then arranged so that 200 or 400 volts might be impressed 

 upon the coils. With 200 volts a deflection of from 5 to 10°^°^ 

 was obtained, which, however, did not reverse with a reversal 

 of the magnetic field ; this deflection was towards a position 

 which the beam tended to take when under the influence of 

 the magnetic field alone. With paratfin as the dielectric the 



