ON THE MEASUREMENT OF TEMPERATURE. 
115 
volume of the dead-space was thus somewhat greater than that determined by the 
method just described, it was necessary to apply a correction amounting to 
where A is the distance between the surface of the mercury and the point, and iris 
the radius of the tulie (see p. 121) ; tti'~ = 0‘636. 
As will be shown later, it was necessary in measuring the pressure to apply 
a correction for the heights of the mercury meniscus in the dead-space and mano¬ 
meter tnbe of the thermometer, which varied slightly in every experiment. If the 
meniscus in the dead-space be considered to have the form of a semi-ellipsoid of 
height h and of radius r, the change of volume of the dead-space with change of height 
of the meniscus will be given by [li — h'). The mean height of the meniscus 
was found to be 1'25 millims., and the extreme variation for any observation never 
exceeded 0‘2 millim., which corresponds to a change of volume of O'OOI cub. centim. 
in the dead-space. As sucli an error bad a considerable effect on the value of P,, and 
Pioo, we were obliged to take it into consideration. 
9. The Stem. 
In our earlier experiment we had found that, if by any accident the mercury 
reached the top of the dead-space, minute globules of it adhered to the glass and 
were in time carried over into the bulb. As the vapour pressure of mercury is 
0-27 millim. at 100° C., this gave rise to serious errors in our measurements. 
To obviate tliis difficulty we introduced into a portion of the stem between c and 
d a closely-wound coil of silver wire 15 millims. long and slightly over 2 millijus. in 
diameter. The tube was made slightly larger at this })oint. Tlie volume of the air¬ 
space was easily determined by weighing the tubes empty and full of water-. 
(a.) 
d-) 
tVeis;ht of stem filled with water . 
10-4.304 
10-4306 
,, ,, em2ity .... 
10-3630 
10-3633 
,, water. 
0-0668 
0-0674 
Volume of stem . . . . 
. 0-0671 cul 
). ceiitim. 
10. ^'olume!i of the Bulb, Stem, and Dead-S'pace. 
Wherr the irrstrunreirt was set rrp perrrranently and the nrercirry br-ought close to 
the point in the dead-space, the whole of the iirterrral space’was considered as divided 
into three regions. 
I. The bulb and stem to within 40 millims of the horizontal portion. 
Volume at ice-point = 94-2096 cirb. centims. 
Volume at boiling-point = ] 94-2096 (T -f 0-000285t) -f 0-0017 
1^ — P 
^ 1 on 
■ cul 
a centims. 
o 
760 
