166 Dr. J. Kerr on Measurements 



unmoved from morning till night. I determined therefore to 

 make a new start, and to devote one day to a small number 

 of independent and very careful measurements, through a 

 pretty long range of potential. 



10. First Appearance of the Law. — Accordingly on one day, 

 Tuesday, the 12th of August, I effected eight measurements 

 of potential and corresponding optical effect. Every precau- 

 tion was adopted, every check and test that I could think of 

 as likely to be of use. All the pieces were placed and fixed 

 with scrupulous care. No guidance towards the results was 

 accepted in any case, except from the actual indications of the 

 instruments. The limits of error were drawn always as close 

 to each other as possible ; and every observation made by an 

 assistant at either of the instruments was repeated and verified 

 by myself before the result was recorded. The work was very 

 tedious, some of the single measurements taking more than an 

 hour of time ; but the results were understood to be propor- 

 tionately sure. 



When the day's work was done and the laboratory closed, 

 I collected the results, and reduced the readings of the com- 

 pensator, subtracting the scale-reading of the true zero-point, 

 and expressing the remainders in terms of a convenient unit, 

 which was the twentieth part of one screw-step of the instru- 

 ment*. The final results were as follows. The numbers in 

 the first line are the scale-readings of the electrometer, or the 

 potentials of the upper conductor ; and those in the second 

 line are the corresponding quantities of optical effect, each 

 probably true to ±1. 



30 40 50 60 80 90 100 110 

 12i 22 36 50 88 115 144 170 

 Observe the effects corresponding to potentials 30 and 60, 

 exactly as 1 to 4 ; the effects again for potentials 40 and 80 ; 

 those also for potentials 50 and 100. It was from these num- 

 bers, exactly as they stand, that I obtained my first view of 

 what I was looking for — a definite relation between potential 

 and optical effect. It will be apparent immediately, that the 

 exactness of equality of those three ratios to each other and to 

 J is partly accidental ; but we shall see no reason to question 

 the truth of the law that is here so very clearly indicated. 



11. It appears therefore that, in the circumstances of the 

 present experiments, the dielectric (CS 2 ) being given, and the 

 electro-optic field also given in form and dimensions, the 

 quantity of optical effect is proportional to the square of the 

 difference of potentials of the two conductors that limit the field. 



* The unit of potential, here and afterwards, is one screw-step of the 

 electrometer, the 200th part of the range of the instrument. 



