1912] Heat Problems. 579 



WEEKLY EVENING MEETING, 



Fi'idav, January 19, 1912. 



His Grace the Duke op Northumberland, K.G. P.O. 

 D.C.L. LL.D. F.R.S., President, in the Chair. 



Professor Sir James Dewar, M.A. LL.D. D.Sc. F.R.S. M.R.I. 



FuUerian Professor of Chemistry, R.I. 



Heat Problems. 



[abstract.] 



The approach to the absolute zero has rendered the problem of the 

 determination of extremely low temperatures a very important one. 

 "Within certain limits resistance thermometers or thermo-couples 

 give satisfactory results ; but now that a temperature within 3° of 

 the absolute zero has been attained, other methods must be resorted 

 to, and the only reliable one is by the measurement of changes at 

 constant volume due to temperature in the pressure of a gas under 

 less than one-third of an atmosphere. The limits of temperature 

 measurable by this method vary with different gases ; helium, for 

 example, giving accurate records to within 3° of the absolute zero. 



Gas Thermometers. 



A gas thermometer for lecture illustration is shown in Fig. 1. 

 A is a copper sphere of a capacity of about 100 c.c, filled with pure 

 hydrogen at a pressure of 273 mm., while immersed in melting 

 ice. The free space, which is the volume of the uncooled narrow 

 tubes required for connection to the manometer, is in this case so 

 small that it may be neglected. A is placed in a metallic vacuum 

 vessel B, containing liquid oxygen, and connected by a thick-walled 

 narrow lead tube to a mercury manometer C. The apparatus is 

 mounted on a movable frame D, so that the height can be ad- 

 justed for projection of the mercury levels. The change of pressure 

 due to cooling the hydrogen in A is shown by the alteration in 

 the levels of the mercury in the manometer. Liquid oxygen re- 

 duces the pressure to 90 mm., corresponding to 90° Abs., and if the 

 liquid is exhausted through E, a pressure of 65 mm. is obtained : 

 i.e. the temperature of the sphere is now G5° Abs., because with this 

 sphere fillet! as described the difference of mercury-levels in milli- 

 metres is practically equal to the temperature oi the sphere in 

 degrees absolute. 



ToL. XX. (No. 106) 2 Q 



