516 



Prof. A. W. Porter on tl 



te 



Experimental. 



It was considered to be of value to determine the actual 

 temperatures acquired by the different portions o£ the wire. 

 For this purpose a wire was prepared with potentiometer 

 leads A, B, C, D, arranged as in fig. 1. 



Va 



Fig. 1. 



Vb Glass V c Glass 



Vd Glass 



B 



The two short lengths o£ glass-covering eliminate the end- 

 effects of the coated portion. The distance from A to B is 

 made as nearly as possible equal to that from C to D. The 

 differences of potential V A — V B and Vc— V D when various 

 steady currents (measured by a shunted Sullivan galvano- 

 meter) flow in the wire were determined by means of high 

 resistance galvanometers which were calibrated for the pur- 

 pose. The temperature coefficient of the platinum wire was 

 determined, also the value of V A — V B corresponding to a 

 measured small current — the wire being immersed in oil — 

 in which case the wire was assumed to be at the temperature 

 of the oil. From these data the temperatures of the two 

 portions of the wire were calculated. There is no need to 

 give the readings as only rough values were aimed at. 

 The temperature-excesses (above 18° C. which was the room 

 temperature) are given in Table IV. 





Table IV. 





Uncovered. 



Covered. 



Baiio. 



832 



152 



55 



702 



117 



6-0 



112 



32 



3-5 



87 



15 



5-8 



The radius of the bare wire was '015 cm. and of its covering 

 •235 cm. 



Great as the ratio of excess temperatures is, it is not nearly 

 as great a number as the "simple theory" requires, viz. 15. 

 Still less does it come up to the requirements of the theory 

 when allowance is made for resistance change. Similar 



