484 Mr. L. C. Jackson on tlie Dielectric Constants 



case to prevent any change in the frequency of oscillation of 

 the two circuits being caused by external disturbances or the 

 movements of the observer. This precaution was quite 

 necessary for accurate work on account of the well-known 

 delicacy of the arrangement. It was also necessary to keep 

 the whole apparatus dry throughout. 



The procedure of the experiments was as follows : — The 

 variable condenser C x was adjusted until the beat note fell 

 to zero when the experimental condenser was removed from 

 the outer end of the leads. The condenser C 2 with air as 

 dielectric was then attached and the process repeated. The 

 air was then replaced by the substance to be tested, and the 

 whole, which was contained in an ordinary boiling-tube, was 

 placed in a bath of liquid air. The test substance solidified 

 (the substances used in the work being all liquid at room 

 temperatures), and was allowed to remain in the liquid air 

 until a constant state was obtained, and the condenser Ci 

 adjusted as before. 



The shifts of the condenser Ci gave first the capacity of 

 C 2 with air as the dielectric, and secondly with the test 

 substance, the ratio of the two shifts (after being suitably 

 corrected) being the value of the dielectric constant of the 

 test substance at the temperature of the liquid-air bath and 

 for the particular frequency of the oscillations used. 



The temperatures were measured throughout by means of 

 a platinum resistance thermometer, and the frequency of the 

 oscillations with the aid of, a wave-meter. 



Care was taken to make the measurements of the capacity 

 of C 2 with air and with the test substance as dielectric under 

 as nearly as possible identical conditions (except as to tempe- 

 rature — see below), and the effect of the leads was eliminated 

 by keeping them permanently attached to Ci so that the 

 difference of any two readings on Oi did not contain any 

 contribution from them. 



The scale of the variable condenser having been previously 

 calibrated, a correction maybe applied to the observed values 

 for the known unevenness of the scale. The fact that the 

 capacity of C 2 with air as dielectric was measured at ordinary 

 temperatures and with the test substance at the temperature 

 of the liquid-air bath, necessitated a correction for the 

 temperature change of the dimensions of the condenser. 



Quite recently a number of methods for the application of 

 u valve" circuits to the measurement of capacities have been 

 described by several investigators : arrangements similar to 

 the above have been proposed by J. Scott Taggart ('Elec- 

 trician,' lxxxii. pp. 466-467, Apr. 18, 1919) and J. Herweg 



