( 589 ) 



Physics. — "On the measvr(>inent 0/ vert/ low temperatures. XIX. 

 Derivation of the pressiwe coefficient of helium for the inter- 

 national heliumthermo meter and the reduction of the j'eadim/s 

 on the heliumthermometer to the absolute scale". Communi- 

 cation N". lOS'' from the Ptiysical Laboratory at Leiden. By 

 Prof. H. Kamerlingh Onnes. 



§ 1. Pressure coefficients of helium. As the absolute zero is known 

 with siifKicient accuracy — from the Leiden observations on hydrogen 

 may be derived 7ooo. = 273°. 10 K. "') a value which, because it 

 agrees with other determinations, is probably not far from the 

 true one — we may by means of the virial coefticients Ba for 

 helium at 0° C. and 100" C, determined in the preceding communi- 

 cation (102"), derive the pressure coefficients of helium at different 

 densities for this range of temperature. For the pressure coefficient of 

 the international helium thermometer ^) i.e. the mean relative pressure 

 coefficient from 0'' C. to J 00° C. for helium with the density 



ro° !■.— 100° c.-| 

 belonging to the zero pressure of 1000 in.m. L «*' J/ or for 



shortness /«,., the formula 



Aa, X 0.36617 4- {Ba,ioooc. - Bao^o) ^^^ 



100 . m„ — 



76 



yields 



100 



%, = 0.0036613. 



(1) 



If one considers that according to table II of Comm. N". 102« 

 the isotherm of 0° gives rather large values for Obs. — Comp., 

 then it seems that the isotherm of 20° C, where the Obs. — Comp. 

 are only small, are more reliable for the derivation given above. 



Ï) In Comm. N". 101'> the value 273^08 is found, but as will be explained in 

 Comm. N". 102', an erratum to Comm. W. 91 Ij XV, the pressure coefficient 

 0.0036627 for hydrogen at 1090 mM. must be restored instead of 0036629 which 

 was derived in the above mentioned communication and used for a certain time. 

 It is to be noted that the difference introduced by this recalculation is not greater 

 than the otlier observational errors. The small differences between some numbers 

 of this communication with the Dutch text are the consequence of this correction. 



3) The scale of the hydrogen thermometer of constant volume at 1000 mm. 

 zero pressure is generally called the scale of the normal hydrogen thermometer 

 (this was also done in Comm. N'^. 97*). As 0^ C. and 760 m.m. are accepted as 

 the normal state for gases, it seems to me preferable to call the scale just mentioned 

 the scale of the international hydrogen thermometer. In the same way we must 

 Speak of the international helium thermometer. 



