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Physics. — ^'On the measurement of very low temperatures. XIII. 

 Determinations with the hydrogen thermometer" Bj Prof. H. 

 Kamerlingh Onnes and C. Braak. (Communication N" 95^ 

 from the Physical Laboratory of Leiden). 



\ 1. Introduction. 



The results of determinations of low temperatures made with the 

 hydrogen thermometer, which was described in Comm. N°. 27 (June 

 1896) and more fully discussed in Comm. N". 60 (September 1900), 

 have already frequently been used, but no further particulars 

 have as yet been given about these determinations themselves. We 

 give them now in connection with a series of observations made in 

 1905 and 1906. They have served for the investigations described in 

 Comm. Nos 95« and 95^^ (June 1906) and' further for determinations 

 of isotherms of hydrogen at low temperatures, which will be discussed 

 in a following communication. Comprising also measurements on liquid 

 hydrogen, they extend over the whole of the accessible area of the 

 lower temperatures. All the precautions which proved necessary in 

 former years, have been taken. The temperature of the bath, in 

 which the thermometer was immerged, could be kept constant 

 to 0°,01 at all temperatures. It was therefore to be expected, that 

 the accuracy and reliability aimed at in the arrangement of the 

 thermometers, might to a great extent be reached.^) In how far this 

 is really the case, the following data may show. 



§ 2. Arrangement of the thermometer. 



There is little to add to Comm. W. 60. The steel capillary con- 

 necting the thermometer bulb and the manometer, was protected from 

 breaking by passing a steel wire along it, the ends of which are 

 soldered to copper hoods, which may be slidden on the steel pieces 

 c and e (Plate II, Comm. N". 27) at the end of the capillary. The 

 dimensions of the thermometer reservoir of Comm. N°. 60 (80 c.M'.) 

 did not present any difficulty in our measurements, the bath in the 

 cryostats (see Comm. N«s. 83, 94^, 94^ and 94/ (May and June 

 1905 and June 1906)) offering sufïicient room besides for the 

 thermometer and other measuring apparatus, for the stirring appa- 

 ratus, which works so thoroughly, that no variation of tempe- 



1) A complete example of the determination of very low temperatures with the 

 hydrogen thermometer was as yet not found in the literature. Such an example 

 follows here. 



