172 



Sir W. Siemens. 



[Apr. 26, 



Table IV. 

 Wire the same as in III. 

 Current decreasing. 



Tem- 

 perature 

 of the 

 room. 





Volts. 







Corre- 

 sponding 

 temperature 

 of wire. 



63° F. 



25 101 



28-31 



710-61 



1-1278 



1240° 



» 



23 '016 



25 33 



582 -99 



1 -1005 



1160 





18-578 



18-327 



340-48 



•9864 



960 





16 "997 



15 "794 



268 '45 



"9292 



o7o 





15 -098 



13 -410 



202 -47 



•8882 



775 



3> 



12 -796 



10-132 



129 -65 



•7918 



605 





11-06 



7-599 



84 -044 



•6870 



440 



JJ 



9-454 



5-662 



53-530 



•5988 



295 





7 513 



4-097 



30-780 



•5452 



180 





6-507 



3-278 



21 -330 



•5037 



130 



3J 



5-04 



2-384 



12 -016 



•4730 







3-217 



1 371 



4-407 



•4258 





65° 



26-856 



31-29 



840-33 



11651 



1290 



The results given in the four tables are plotted out on the curve 

 marked (A). The abscissse give the rate at which the energy of the 

 current is converted into heat, and the ordinates the corresponding 

 resistance of the wire. 



To determine the temperature of the wire corresponding to each 

 resistance, another series of experiments was made, which are 

 described hereafter. The values of a, |3, and 7 obtained were — 



*=0-0119 -] 

 (3=0 -00112 V 

 7=0 512 J 



hence -0119T*+ '00112T+ -512; 



where r is the resistance of the wire at the freezing point. By giving 

 to T various values in this formula, a curve can be constructed 

 showing the relation between the resistance and absolute temperature. 

 Such a curve was drawn, and approximated for high temperatures to 

 a straight line, as evidently must be the case from the form of the 



equation. By solving the equation for the maximum value of — 



observed, it was found that the temperature of the wire when bright 

 red hot was about 1100° C. It is known that platinum wire melts at 

 approximately 1800° C. 



The curve of relation between the temperature of the wire and the 



