410 DR. C. H. LEES ON THE THERMAL CONDUCTIVITIES OF SOLIDS 



of copper wire to get rid of thermo-electric forces in the circuit, T, other than those to 

 be measured. 



The galvanometer enabled the determinations to be made to about '003 ohm. By 

 means of the mercury cups (fig. 7) any one of the thermo-electric circuits could 

 be connected through the galvanometer' to RI, and the electromotive force- in it 

 determined, or by arranging two of the circuits in opposition, the difference of electro- 

 motive forces in any pair could be found. 



Standardisation of Thermo-couples. 



To determine the constants of each of the thermo-couples, the various copper 

 discs were bound together by tape, a thin sheet of mica being placed between 

 each disc for insulation, and the bulb of a thermometer graduated in ^ degrees was 

 placed in contact with, and bound to, the discs by a few turns of the same tape. The 

 combination was then placed in its usual position in the air bath, and the bath heated 

 by sending a current through the coil encircling it. When the temperature of the 

 bath had been raised to about 50 C., the current round it was diminished till the 

 temperature ceased to rise, and further adjusted till the temperature remained 

 constant. After about half-an-hour the indications of the thermometer in the bath, 

 and of a similar thermometer in the mercury cups in which the other junctions were 

 placed, were read, the electromotive forces in each therm o-circuit found, and the 

 thermometers again read. After a few minutes' interval the process was repeated. 



Since iu the test of conductivity it is the difference of the temperatures of the discs 

 which is required, observations of differences of the electromotive forces in the various 

 circuits were in each case taken. As these differences were small, and could be 

 observed with the same degree of accuracy as the actual electromotive forces, which 

 were comparatively large, the calculations of conductivity have all been based on 

 them, one of the observations of actual electromotive force having been used to fix 

 the mean temperature of the disc experimented on. 



The following table gives the results of the observations : 



6 = the temperature of the discs in the bath. 



= that of the mercury cups. 



U = the number of ohms of the potentiometer necessary to give a balance in the 



circuit from the "upper disc." 1 ohm corresponds to 1 '434/30, 000 



= '000044 volt. 

 U M = the ohms giving a balance when " upper " and " middle " circuits were 



opposed. 



U L = ditto . . . ditto, when " upper " and " lower " opposed. 

 B = ditto . . . ditto, when thermo-junction from inside surface of air bath was in 



circuit. 



