300 Profs. J. A. Fleming and J. Dewar. On the 



inside at the top, and 26 cm. inside diameter at the bottom. The 

 bottom of the outer cone is closed by a brass plate fastened on with 

 six set screws, and which plate carries a small ebonite steady pin 

 1*5 cm. long and 3 mm. in diameter, screwed into the centre. In 

 some instances a quartz pin is preferable, and can be used. 



The inner cone is also closed at the bottom 

 and open at the top. The tw T o cones were accu- 

 rately ground together and, after finishing, were 

 thickly gilt. 



The inner cone is 12*7 cm. in length and 4*8 cm. 

 in diameter on the outside at the top, and 2*6 cm. 

 across the bottom. The bottom of the inner cone 

 is closed by a thick plate having a recessed hole 

 in it, into which the above-mentioned ebonite 

 pin fits easily, and acts as a steady-pin to keep 

 the two cones coaxial. To the top of the inner 

 cone is fixed, by means of a three-spoke carrier, 

 a vertical screw pin, and this screw passes 

 through a metal boss in the centre of an ebonite 

 bushing piece fixed in the middle of another 

 three-spoke carrier fastened by three removable 

 screws to the upper edge of the outer cone. 



By means of a nut on the screw, the inner 

 cone can be displaced coaxially with the outer 

 cone, so as to leave between the cones a conical 

 space into which any fluid can be introduced. 

 A lock nut enables the inner cone to be fixed. 



The inner and outer cone then form the two 

 plates of the condenser, and the fluid, or frozen 

 fluid in the conical interspace, the dielectric. 

 The total surface of the inner cone is 147' 6 

 sq. cm. The cross section of the ebonite 

 steady-pin is, therefore, only about one fifteen- 

 hundredth of the total exposed surface of the 

 condenser, and any correction for its dielectric constant may be 

 neglected. The weight of the whole condenser is 960 grams. 



The thickness of the walls of the two brass cones is about 2 mm. 

 in each case. 



It will be seen, therefore, that if any fluid is introduced between 

 the cones and frozen, any expansion thereby taking place lifts the 

 inner cone, but does not in any way deform the condenser. If, then, 

 on cooling, a subsequent contraction takes place, the weight of the 

 inner cone makes it fall back and follow up the shrinking dielectric. 

 And to assist this a small spiral spring is placed around the 

 central screw between the three-spoke carriers attached to the inner 



Fig. 1. — Electrical cone 

 condenser for low tem- 

 perature measurement 

 of dielectric constant. 



