"228 Mr. W. Makower on a Determination of the 



was a wire x passing out through a loosely fitting cork c, 

 through which also passed a glass tube with a platinum 

 wire p sealed through it, electrically connected to w. (This 

 was employed in the chronograph measurements to be de- 

 scribed below.) When the pressure in the " air-vessel " was 

 equal to the atmospheric pressure, the wire w was in elec- 

 trical connexion with the wire x ; on raising the pressure 

 the contact between the wire iv and the mercury was broken, 

 thus breaking the electric circuit from the wire w through 

 the mercury to the wire x. If the pressure in the " air- 

 vessel n was now suddenly released, contact was made between 

 the mercury and the wire w after a definite time had elapsed. 

 This time (which we will denote by r) could be varied at will 

 by raising or lowering the limb containing the wire x, and 

 also by means of a screw pinch-cock (not shown in the 

 figure) which served to constrict, to a greater or less ex- 

 tent, the rubber tubing joining the two limbs of the gauge. 

 The two wires x and w were connected respectively to the 

 two terminals of the key K, thus putting the gauge in 

 parallel with this key. 



3. Measurement of Temperature. 



From formula (1) it appears that it is necessary to measure 

 both the temperature (0i) before opening the vessel and the 

 temperature (0 2 ) to which the gas has fallen, measured at an 

 instant as soon as possible after opening the vessel, as the gas 

 begins to heat up, by conduction from the walls of the vessel, 

 almost at once after releasing the pressure. In order, there- 

 fore, to obtain reliable results it is necessary that the ther- 

 mometer which is used should be able to follow as nearly as 

 possible the variations of temperature of the gas. On this 

 account a platinum thermometer of a pattern similar to that 

 employed by Callendar in his steam-engine experiments of 

 1895 was constructed. 



A piece of pure platinum wire (p) (PI. I. fig. 2) of diameter 

 •001 inch was soldered * on to the platinum leads / sealed 

 through one end of the glass tubes g : these in turn were 

 soldered on to the copper leads L, passing out of the glass 

 tubes through the other ends which were left open. Close 

 to the thermometer leads were placed compensating leads to 

 which were soldered a piece of fine platinum wire p' of the 



* In the air-experinients ordinary soft solder was used. In the steam 

 experiments to he described below the line platinum wires were attached 

 with silver solder. 



