of some of the JEJsters at Low Temperatures. 483 



dielectric constant o£ the substance under test for the parti- 

 cular conditions of* temperature and frequency of oscillation 

 obtaining at the time of the experiment. 



The actual arrangement by which the above-described 

 method was applied to the investigation of the dielectric 

 constants of the esters is shown diagramatically in fig. 1. 

 It consists of two simple "valve" circuits placed side by 

 side, so that, as in the heterodyne method of reception, a 

 beat note is heard in the telephone placed in one of the 

 circuits. In parallel with the variable condenser C x is placed 

 the experimental condenser C 2 , the change of the capacity 

 of which with change of dielectric is to be determined. The 

 circuit 2 remains untouched throughout the experiment, and 

 serves as a means whereby the frequency of oscillation of 

 circuit 1 can be brought to some standard value whenever 



Fig. l. 



\x\*\> 



r 



^j 



c 3=r Ci ^ 







\fr 



HI 



H.T. 



H.T. 



required. The method actually adopted for this purpose 

 was to adjust Ci so that the beat note in the telephone 

 always just fell to zero. The accuracy with which the scale 

 of the variable condenser could be read determined the 

 accuracy with which it was necessary to adjust the beat note 

 to the standard value, it being found that the method of ad- 

 justing to zero gave all the accuracy of setting required, so 

 that the more elaborate and accurate method of comparing 

 the beat note with a standard fork was not used. 



The experimental condenser was a brass cylindrical one, 

 consisting of four coaxial tubes connected together alternately 

 and fastened at one end into a fibre separator. The di-imeter 

 and length of the outer tube were 2*06 cms. and 12*36 cms. 

 respectively, this size being such that the condenser would 

 just slip into an ordinary boiling-tube. 



The whole of the two circuits, with the exception of C 2 

 and the telephones T, was enclosed in an earthed tin-plate 



212 



