Second and Third Order Tests of the "Mther Drift" 79 



planes of the condenser, and c the principal axis of the sensitive 

 strip. Similarly at A'', h lz p l3 a 1? c x are new positions of these 

 elements due to any constant rotation of all of the elements of 

 the second system except k! '. If now there be any damping 

 of the electric oscillations, the effect of the one condenser on 

 the polarized ray for, say, the first oscillation, may be made 

 equal to that of the other condenser for the second oscillation 

 by such a rotation, and so on. Thus these two condensers 

 become an equivalent " crossed" system, the one compensating 

 completely the effect of the other, even if the electric stresses in 

 each are not the same. This would hold for electric oscillations 

 which produce successive stresses that are in a constant ratio 

 to each other. If now the interval of passage of the beam 

 of light between k and k' is any multiple of a half period 

 of the vibration, we may obtain compensation, and hence 

 retain the settings for a match in the two half-shade systems 

 c and & '. If there should be any difference in the interval 

 of passage in the two opposite directions, we should not 

 obtain a match in the one if we set for the same in the other, 

 after the frequency had been varied so as to give this exact 

 multiple of the half period in this latter system. This 

 would be determined by noting when the intensity of the 

 field approximated a minimum intensity. 



For low frequencies sunlight has been used ; but for the 

 higher frequencies this source has not given satisfactory 

 results on account of the very brief duration of the spark in 

 the exciter, and, consequently, the integral time of the electric 

 stresses due to such a discharge. In this case the spark 

 itself may be used to advantage, since here we have a very 

 much greater intensity during the period of the electric 

 stresses. The greater uniformity and intensity of a vapour 

 spark-gap, e. g. of mercury, recommend its use in connexion 

 with the half-shade system. To maintain greater uniformity 

 in the amplitude of the oscillations, and hence in the Kerr 

 " effect," a secondary or resonance circuit itself, with its 

 condenser system, may be used, as this will give sufficient 

 double refraction in the dielectric to make accurate settings. 

 Such a half-shade system, where the photometric sensibility 

 is as low, say, as 2 per cent., will show a change of phase of 

 0*3 x 10~ 4 X*. Since now the maxima of the Kerr " effect" 

 occur every half oscillation, the distance between the condenser 

 k and k' needs only to be equal to the space traversed by the 

 ray in half an oscillation, or rf=^L, where L is the wave- 

 length of an electric oscillation. This would give us a 

 change of phase in the one direction of 0*3 x 10 " 4 \ and an 



* Phys. Rev. vol. xviii. p. 85. 



