90 Mr. D. M'Farlane on the Surface- conductivity [Jan. 11, 



January 11, 1872. 

 GEORGE BIDDELL AIRY, C.B., President, in the Chair. 



The following communications were read : — 



I. c< Experiments made to determine Surface-conductivity for Heat 

 in Absolute Measure." By Donald M'Farlane. Commu- 

 nicated by Sir William Thomson, F.R.S. Received August 8, 

 1871. 



The experiments described in this paper were made in the Physical 

 Laboratory of the University of Glasgow, under the direction of Sir Wil- 

 liam Thomson, during the summer of 1871. A set of similar experiments 

 were made in 1865 ; but being merely preliminary, carried on by different 

 individuals, and embracing only a limited range of temperatures, it is 

 thought unnecessary to allude further to them here*. 



A copper ball, 2 centimetres radius, having a thermo-electric junction at 

 its centre, was suspended in the interior of a double-walled tin-plate vessel 

 which had the space between the double sides filled with water at the 

 atmospheric temperature, and the interior coated with lamp-black. The 

 other junction was in metallic contact with the outside of the vessel, and 

 the circuit was completed through the coil of a mirror galvanometer. One 

 junction was thus kept at a nearly constant temperature of about 14° Cent., 

 while the other had the gradually diminishing temperature of the ball. 



Having adjusted the galvanometer to the degree of sensitiveness desired, 

 the copper ball was heated in the flame of a spirit-lamp till its tempera- 

 ture was considerably above that required to throw the spot of light off the 

 scale ; it was then put into position in the interior of the tin-plate vessel, 

 and as soon as the spot of light came witbin range, the deflections from 

 the zero position were noted at intervals of one minute exactly till the 

 change of deflection was reduced to about two scale-divisions per minute. 



Two series of experiments were made in this way, each consisting of 

 several sets of readings. In the first the ball had a bright surface, and in 

 the second it was coated with a thin covering of soot from the flame of a 

 lamp, and in both the air was kept moist by a saucer containing a quantity 

 of water placed in the interior of the tin-plate vessel. 



As the range of differences of temperatures of the junctions extended 

 over 50° Cent., the change in the difference of thermo-electric qualities of 

 the copper and iron wires forming the junctions was very considerable, 



* These experiments consisted of two series, one with the air moist by a little water 

 placed in the interior of the vessel, the other having the air dried by substituting sul- 

 phuric acid for the water in the first ; and the results in the two cases were so nearly alike, 

 that any effect due to the moisture or dryness of the air could not be distinguished from 

 errors of observation. Fvom this circumstance, as well as the limited range of tempe- 

 ratures, these results are not given here. 



