326 Profs. J. A. Fleming and J. Dewar. On the Dielectric 



Ethylene dibromide, therefore, presents a remarkable contrast to 

 water or glycerine. Its dielectric constant at ordinary temperature, 

 15° C, has a value of about 5. 



It falls, on freezing, to nearly 3, and it continues, as seen by the 

 above table, nearly constant down to the lowest temperature reached, 

 at which it becomes 2*7. 



Its resistance also remains, when cooled down after freezing, 

 exceedingly high and constant, the condenser resistance being over 

 10,000 megohms between —200° and -30°. 



From a consideration of the values which have been found for the 

 dielectric constants of various bodies, it appears that the presence in 

 a molecule of certain radicles such as hydroxyl and nitryl always 

 cause high values of the dielectric constant in the liquid state. 

 Hence we have examined one well-known nitro-substitution com- 

 pound, nitrobenzol, to determine how this value is altered by very 

 low temperatures. The dielectric constant of mononitrobenzol is 

 given by C B. Thwing as 32*19, presumably at 15° C. Our values 

 for its dielectric constant at low temperatures and when solidified 

 are as follows : — 



Dielectric Constant of Nitro-benzol. 



Frequency of contact maker 120. Galvanometer deflection with air 

 as dielectric in condenser = 4'15 cm. for 100 volts. 



Temperature 

 in platinum 

 degrees. 



Galvanometer 



deflection 

 in centimetres. 



Dielectric 

 constant. 



-205-2 



11-45 



2-61 



-205-0 



11-55 



2-64 



-191-2 



11-15 



2-54 



-175-9 



1115 



2-54 



-157-5 



11-25 



2-57 



-141-3 



11-25 



2-57 



-124-5 



11-38 



2-59 



-103-3 



11-5 



2-63 



-93-8 



11-6 



265 



-84-0 



2-4 



2-97 



-38-8 



2-73 



316 



-215 



2-92 



3-40 



Observations. 

 Charging voltage 101 "0 



Charging voltage 20*0 



The values of the dielectric constant of glycerine, ethylene dibrom- 

 ide, and nitrobenzol are plotted with some other substances as curves 

 in fig. 2, and it will be seen that those of the ethylene dibromide 

 and nitrobenzol values are practically constant over a wide range of 

 temperature, and very nearly equal to that of ice at —200°. 



