BETE 
purpose we used the open standard manometer *), since the pressures 
were all below 12.80 atm. (critical pressure) and in our closed 
hydrogen-manometer, which is otherwise much simpler in use, the 
mercury does not become visible till 20 atmospheres. 
To the results of these measurements and the use made of them 
in the calculation of the corrections we shall return in diseussing 
the calculations. 
The temperature was measured and at the same time kept constant 
by means of two platinum resistance thermometers Pt, and Pty, 
which were compared with a helium thermometer shortly after the 
measurements and under exactly the same conditions, i. e. in the 
eryostat and at the same temperatures as had been used in the 
diameter-measurements. The agreement of the two thermometers 
was completely satisfactory. *) 
The temperature to be taken for the glass capillary of the dilato- 
meter was this time separately determined with a simple gas ther- 
mometer in the form of a tube which was mounted beside the 
capillary in question in the lid of the cryostat, and had been 
similarly used in previous measurements *). In this manner the 
temperature of the capillary is measnied in each determination 
separately, instead of making an estimation or deriving it from 
previous determinations; the method also simplifies the calculation 
of the corrections, since only one temperature is dealt with, whereas 
before the capillary had as a rule to be divided into three portions 
each with its own mean temperature *). 
A few days before the commencement of the real measurements 
a “general rehearsal’ was held, chiefly to ascertain the degree of 
constancy and uniformity of the temperature to be obtained with 
the new cryostat, and the influence of various methods of illumi- 
nation on the temperature. The filling of the dilatometer with liquid 
hydrogen was found to proceed without a hiteh. 
During the period of the measurements the illumination trials 
were continued for the first ten days: a metal wire lamp behind 
a vessel with alum solution was found to give no disturbance ; with 
clear weather diffuse daylight was also sometimes used. 
1) H. KAMERLINGH ONNES, these Proceedings 1, p. 213, Leiden. Comm. N°. 44. 
*) Comp. fig. 2 of the next communication. 
3) For instance by P. G. CatH and H. KAMERLINGH ONNEs, these Procee- 
dings 20, p. 991, p. 1155, Leiden. Comm. N°. 152a and P. G. Catn, Disser- 
tation, Leiden 1917, Figure on p. 101. 
*) F. HENNING. Die Grundlagen, Methoden und Ergebnisse der Temperatur- 
-messung (1915) pp. 46 and 47. 
76 
Proceedings Royal Acad. Amsterdam. Vol. XXIII. 
