( 496 ) 
measurements of temperature by the hydrogen thermometer, this 
is not of much use as long as we are not correspondingly certain 
of the equality of the temperature in the resistance and the 
hydrogen thermometer. This equality was obtained by immersing the 
hydrogen thermometer together with the resistance to be compared 
into the bath of liquified gas in the eryostate (described in Comm. 
54). Therefore in the construction of the resistance thermometer and 
in the arrangement of the measurements of resistance the circum- 
stances under which the resistance is employed are taken into account. 
In order to bring the platinum wire and the hydrogen thermo- 
meter as much as possible under the same conditions, the former 
was wound round a hollow cylinder, into which the reservoir of the 
hydrogen thermometer was placed. This form of the resistance is 
also a very appropriate one to measure the temperature of the same 
vessel, when other apparatus than the hydrogen thermometer occupy 
the place usually occupied by the latter. Even with a high resistance 
a thermometer of this form occupies a very small space, and the 
remainder of the vessel is thus left very satisfactorily free for other 
experiments. 
In order to obtain reliable measurements with the hydrogen thermo- 
meter, the latter is kept for a long time at a constant temperature 
and the operations required for this are simplified when the wire 
follows. directly fluctuations or variations of the temperature of the 
vessel, even when they are very small, and indicates them in the 
reading fof the galvanometer. This is obtained by bringing the wire 
into immediate contact with the liquefied gas. Also the heat due to 
the Joure effect and — a question to which the proper attention 
has not always been paid — the influence of radiation and of conduction 
of heat along the leads are diminished by this as much as possible. 
There is no objection to using a naked wire in the case of platinum, 
with regard to any action on the substance itself. But the difficulty 
remains that in using naked wires (comp. Commun. n°. 27 § 10) 
care must be taken that the bath into which the platinum resistance 
is immersed should be a better insulator than otherwise. Therefore in 
the construction of the resistance thermometer it was desirable to 
avoid the use of ebonite or similar substances liable to be attacked 
more or less by liquefied gases or by the liquid bath used in the 
determination of the zero. 
This has been obtained very satisfactorily by constructing a 
resistance, exclusively of glass and metal, which even in boiling 
oxygen amounts to 30 Ohms. 
On a glass cylinder (see fig. 1) height 50 m.m., outer diameter 
