230 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



B 



M 



the glass, which must have been initially strained. This led to the 

 adoption of a form in which there were no bends in the glass 



(Figure 6). The glass capillary (A) with 

 the cup on the upper end for an electrode 

 dips into the thin walled tube B containing 

 mercury into which the other electrode 

 dips. This form worked perfectly well, 

 but was somewhat less convenient to 

 handle than the U form. It was finally 

 found that by making the stem of the 

 U capillary very 

 slender, about 

 1.5 mm., there 

 was no tend- 

 ency to crack at 

 the bend, and 

 this was the 

 formwithwhich 

 the final deter- 

 minations were 

 made. 



The U capil- 



KJ 



ing vessel B must be of thin 

 glass to insure freedom from 

 breakage. 



Figure 6. Alternative 

 form of containing vessel for 

 the mercury resistance. The 

 resistance of the thin thread 

 of mercury in the capillary A larv (B, Figure 

 is measured. The contain- y\ j s mounted 



in a split cylin- 

 drical piece of 

 steel (A, Figure 

 7), which is attached to the lower end of 

 the insulating plug. The capillary and 

 plug may thus be connected together and 

 inserted as one piece into the pressure 

 chamber with the certainty that none of 

 the connections will be disarranged in 

 assembling. By making the split steel 

 cylinder containing the U a snug fit, the 

 glass is closely surrounded by metal on all 

 sides, and the quantity of liquid transmit- 

 ting the pressure is greatly diminished. 

 This has the double advantage of decreas- 

 ing the total change of volume of liquid 

 necessary to reach a given pressure, and of decreasing the total heat of 

 compression. The heat of compression generated in the small volume 



Figure 7. Manner of 

 mounting the mercury re- 

 sistance. The steel envelope 

 A speedily conducts away the 

 heat of compression. 



