Section III, 1919 [73] Trans. R.S.C. 



Measurements of Temperature Gradients in Air Between Parallel Planes 

 Maintained at Different Temperatures and their Applica- 

 tion to a Determination of the Temperature 

 Variation of Thermal Conductivity 



By G. H. Henderson, M.A. 



Presented by Dr. L. V. King, F.R.S.C. 



(Read May Meeting, 1919.) 



This paper is a summary of an investigation of the temperature 

 gradient in air at atmospheric pressure between parallel planes main- 

 tained at temperatures differing considerably from one another, with 

 the object of determining the variation of thermal conductivity with 

 temperature over a wide range of temperature. A more detailed 

 account of the investigation will be given elsewhere. 



If we have two infinite parallel planes at different fixed tempera- 

 tures, the quantity of heat Q, crossing unit area of any intermediate 

 plane, is constant when a state of steady flow of heat is reached and 

 is given by Q = K.ddldz,'^ where K is the thermal conductivity of the 

 gas between the plates and ddjdz is the temperature gradient. 



Hence, without determining Q which is difificult to measure, the 

 variation of K with temperature may be determined from a knowl- 

 edge of the temperature gradient alone. 



Apparatus 



The apparatus consisted of a lower plate of copper under which 

 water circulated, and an upper plate of asbestos blocks wound with 

 nichrome wire and heated electrically. Between the two plates a 

 volume of air was enclosed by a glass ring and plate. Temperatures 

 in the air were measured by means of a thermo-element, which could 

 be moved up and down by a screw and whose height above the lower 

 plane was measured with a cathetometer. 



Fig. 1 is a diagram of the apparatus. The glass ring was 1 • 4 cm. 

 high and 7 • 8 cm. internal diameter. The cold resistance of the heating 

 coil was about 5 ohms. The thermocouple was made of 1 mil constan- 

 tan and 1-6 mil copper wires, calibrated in steam, napthalene and 

 sulphur vapours. 



A series of observations was carried out at four different power 

 inputs. At a given power input the number of temperature determina- 



Sec. III. Sig. 6 



