APPARATUS 



*3 



The filling of the simple apparatus was conducted in the following 

 manner. The lower tube of the glass jacket was filled with mercury, 

 a single drop of water was placed on its surface, and the stopper was 

 carefully inserted and tied down with stout string passed over a rubber 

 shoulder. The latter, used to give needed elasticity, is indicated by 

 dotted lines in Fig. 3. The cavity in the stopper was filled with 

 mercury from above by means of a capillary funnel tube, as far as the 

 point of the platinum wire ; and the mercury surface was separated 

 from the mineral oil of the Cailletet compression barrel by water, 

 which filled the upper part of the funnel tube. As already stated the 

 meniscus must be perfectly clear and free from oil, otherwise its curve 

 is irregular. In order to conduct away the heat of compression and to 

 make the lower electrical contact, mercury was poured around the 

 outside of the lower two thirds of the glass jacket. 



The upper platinum wire was connected with an insulated wire 

 running through a capillary glass tube sealed into the upper movable 

 part of the Cailletet apparatus, and the contact was detected with the 

 help of a feeble cell and a delicate index-galvanometer. The appara- 

 tus being tightly screwed into place, pressure was applied until the 

 circuit was broken a condition which showed that the mercury had 

 been] compressed until its meniscus had fallen below the platinum 

 point. The mercury was kept at this point for twenty minutes, or 

 until the pressure readings became constant. The heating effect of the 

 compression was considerable ; but the inner jacket being immersed in 

 mercury, the heat was quickly conducted away to the large surround- 

 ing thermostat, kept constant to within 0.01 C, in which the Cailletet 

 barrel was immersed. An idea of the speed of this equalization of 

 temperature can be gained from the following table. Here the pres- 

 sures corresponding to the break of electrical contact are given at 

 various times, beginning with the time of the first quick compression : 



