250 Prof. E. Wiedemann on the 



The quantity of energy E lost per second by radiation 

 during the constant luminosity of the platinum wire is pro- 

 portional to the product of the resistance w of the wire, and 

 the square of the intensity of the current i, or, if they are 

 measured in ohms and amperes, 



E = 0'24i^' 2 , 



where 0*24 is the necessary factor of reduction to convert 

 energies measured in ohms and amperes into gramme-calories 

 per second*. 



In order to determine these quantities a Wheatstone's 

 bridge (fig. 2) was constructed in the following mannerf : — 



The branch 1 contained a vertical rectangular bar of wood, 

 «, surrounded by oil, upon which various constant known 

 resistances were formed, as well as the reflecting galvano- 

 meter g lf serving to measure the strength of the current with 

 the strong currents which had to be used here. A thick 

 copper ring was employed instead of a multiplier, the resist- 

 ance of which and its connexions was k. 



The branch 2 was formed by the platinum wire d to be inves- 

 tigated, whose resistance at temperature t may be called w t . 



The branch 3 contained a large resistance of y nominal 

 Siemens units, and branch 4 a box of resistance-coils of from 

 1 to 5000 Siemens units, the accuracy of which had been 

 previously tested. 



The heating-current entered at the point 1, 3, and was 

 furnished by six large Bunsen cells arranged three in series 

 (the reason for not employing a Gramme or dynamo is given 

 further on), and was led off at the point 2, 4. A variable 

 resistance included in the circuit at W permitted the variation 

 of the strength of the current, and with it the heat produced, 

 and consequently the luminosity. 



The resistance of the branches 3 and 4 was always so great 

 in comparison with 1 and 2 that the variations of the current 

 caused no perceptible variation in the intensity of the current 

 in 1 and 2. (The resistance of 1 and 2 never exceeded 2 

 Siemens units, and the resistance in 3 and 4 was not less than 

 400 Siemens units.) 



The sensitive galvanometer g 2 serving for the measurement 

 of the resistances was inserted between the points 1, 2 and 3, 4. 

 By changing the plugs in the box of resistance-coils in 

 branch 4 the resistance was altered until the galvanometer 

 remained at rest on opening and closing the key. 



* Kohlrausch, Leitfaden d. pract. Phjsik. 6th ed. p. 334 (1887). 



t Compare Bottomley, Phil. Trans. Roy. Soc. London, cxviii. p. 429 



(1887). 



