Whitehead — Magnetic Effect of Electric Displacement. 119 



proper reversal was obtained to the extent of 4 or S™'" on two 

 occasions, one with a reversal of the electric field, the other 

 with a reversal of the magnetic field. In both instances the 

 deflections were from a zero position constant within 1"'°^, and 

 appeared as a slow motion immediately on closing the switch. 

 The above mentioned disturbing effect of the magnetic field 

 was always evident through all the experiments, but in this 

 form of the apparatus in several instances there were distinct 

 initial deflections in opposite sense to this tendency. 



When the switches remained closed long enough, the above 

 disturbing tendency seemed to predominate regardless of the 

 direction of the field. The question at once arose as to whether 

 it was a magnetic or an electric effect. Experiments with bits 

 of iron filing on the dielectrics, with hard rubber known to be 

 slightly magnetic, mth rock salt which gave no trace of mag- 

 netic impurities, and with both direct and alternating currents, 

 indicated that a part of this disturbing infiuence was due to the 

 difference in the intensity of the magnetic field at the center 

 and close to the side of the coil. For this reason only rock 

 salt, glass and paraffin were used, as they appeared quite free 

 from magnetic impurities ; with them the disturbance was less 

 than with sulphur and hard rubber, though it was still the 

 predominating influence. Further experiments with rock salt 

 several times gave evidences of the proper reversals to the 

 extent of 2 or 3™" ; and with glass on one occasion the expected 

 reversals of deflection were noticeable for six switch reversals ; 

 these deflections were only 2 to 3"""'. Yery long waits between 

 readings were necessary owing to the difficulty of obtaining a 

 steady zero ; this was probably due to air currents set up within 

 the case caused by the heating of the coils when on the 200 

 volt circuit ; the six readings mentioned required one afternoon. 

 On the next day the proper reversals of deflection were observed 

 for four reversals each of magnetic and electric fields ; they 

 were from 2 to 5""". 



It must be stated, however, that these proper deflections 

 were taken from among a mass of attempts which resulted 

 sometimes in no deflection at all and at others in deflections in 

 the wrong sense. While it was sometimes possible to account 

 for these by a steady shifting of the zero position, I still ques- 

 tioned whether I had obtained the effect, and looked to the 

 improvement of the apparatus. 



Second and Third Form of Apparatus. — In the second form 

 the air space was reduced and the electric screening made bet- 

 ter by enclosing only the electrodes and suspended system 

 within the case, the coils being outside. The case was entirely 

 of brass, the posts of the electrodes being set in insulating 

 bushings. To prevent secondary currents in the cylindrical 



