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temperature, ^^hen all over the part between them the external 

 excliano-e of heat, by conduction, convection and radiation is small 

 enouo-h to be disregarded with respect to the other thermal effects. 

 The TnoMSON-constant a may then be expressed : 



iio Th" 



wherein i represents the strength of the current; ?r the resistance; /the 

 mechanical equivalent of heat; q the section of the conductor ; f/the 

 difference of temperature between the two parts of constant temperature; 

 / the distance between those two parts; 2 LtiJI the change of tempe- 

 rature which manifests itself in the middle-section when the current 

 is reversed; and A^?^ the rise of temperature in the same section 



according to Jonle's law. 



In order to be able to measure 4:Lt/,u instead of 2LriiU the 

 mercury was investigated in a f7-shaped glass tube, put in a vertical 

 position, the curved part up. The upper part of this L^-tube was 

 enclosed in a glass bulb, in which different fluids (acetone, water, 

 aniline, glycerin) could be kept boiling by an electric cnrrent. In 

 this way the upper part was kept at a constant temperature. For 

 the same pnrpose the bottom parts of the legs of the f7-tube, 

 which ^vere closed by small rods of platinum, were placed in 

 running tapwater. 



In the parts of non-uniform temjierature this temperature was 

 measured in sections lialfway between the constant limits. If, after 

 the current has been sent through in one direction, there should 

 exist a certain difference of temperature between the two middle- 

 sections, this difference will suffer a change of 4Aja?< by reversing 

 the current, if the condition about the external exchange of heat is 

 fulfilled: 



Therefore the parts of non-uniform temperature were enclosed in 

 a large vacuum-tube, for the greater part of glass, with a brass 

 bottom and, for the sake of practical advantages, the glass boiling 

 bulb and part of the condenser upon it were also enclosed in this tube. 



In order to measure A ii, separate experiments were made, Avith 



as nearly as possible the same current. By making the current 

 go first through one leg and then through the other the diffe- 

 rence in temperature of the middle-sections was varied by 2 Ay/. 



For measuring the temperature in the mercury the thermo-electric 

 difference between this metal and platinum was used. Different kinds 



