256 * Profs. J. Dewar and J. A. Fleming. Dielectric Constant? 



electrical capacity of the cone condenser as then arranged. A 

 standard condenser belonging to the Davy- Faraday laboratory, and 

 having a capacity of one-thousandth of a microfarad, was then sub- 

 stituted for the cone condenser, and gave right and left deflections of 

 19*1 and 19'6 cm. respectively when charged with 97 volts. This 

 corrected for capacity of leads (= 0*4 cm.) and reduced to 100 volts 

 becomes 19 - 53. Hence, since the electrical capacities are proportional 

 to the galvanometer deflection, the electrical capacity of our cone 

 3-58 



condenser is in - D X 001 of a microfarad, or 0*000183 of a micro- 

 farad. 



The capacity of the gilt cone condenser was then again measured 

 with air at 20° C. as dielectric, and the galvanometer deflection 

 observed. The outer cone was then cooled to — 185° C. by quickly 

 applying to it a large quantity of liquid air whilst the inner cone 

 remained at about 20° C, and the galvanometer deflection again 

 observed. This deflection was taken again when the inner cone had 

 fallen in temperature to —75°, and finally when both inner and outer 

 cones were at —185° C. The following numbers giving the galvano- 

 meter deflections are then proportional to the electrical capacity of 

 the condenser between the cones. 



Table V. — Examination of the Effect of Cooling on the Electrical 

 Capacity of the Cone Condenser. 



Galvanometer scale deflection 



in cm. or electrical 

 capacity of the cone condenser 



in arbitrary units, 

 the dielectric being gaseous air. Remarks. 



4"22 cm e , . . Both inner and outer cones both 



at 20° C. 



4-82 cm Outer cone at -185° C. 



Inner cone at 20° C, about. 

 4*58 cm Outer cone at —185° C. 



Inner cone at — 75° C. 



4*19 cm Outer and inner cones both at 



-185° C. 



Hence it is clear that mere change of temperature of the metal 

 work of the cone condenser does not affect its electrical capacity by 

 more than 1, or perhaps 2, per cent., and any larger changes in 

 capacity found on cooling must be due to a real change in the 

 dielectric constant of any dielectric substituted for the air between 

 the cones, and not to mere dimensional changes of the condenser 

 itself produced by the cooling. 



Another matter to which our attention was directed was the 



