metallic Conductors and their Resistance to Electric Currents. 9 



the upper one remaining nearly as before. If the room be completely darkened, 

 this proceeds till only the upper inch of it is visible, and the least additional 

 decrease of ignition makes all disappear, when the pyrometer shows about 

 550'. 



The first used was originally -j^ of an inch diameter; but during the nu- 

 merous preliminary experiments it was stretched till its thickness was only 

 -pi^ : the results obtained are arranged in the following table, each being a 

 mean of ten trials : 



The column headed contains the deflection of the rheometer; the next 

 gives the intensity of the current, its unit being that which deflects the rhe- 

 ometer to 45", and disengages in a voltameter 6.57 inches of gases, at their 

 normal temperature and pressure, in five minutes. The two next columns 

 give P, the reading of the pyrometer corrected for the temperature of the air 

 and r, the temperature of the wire computed from it. The last gives A, the 

 resistance of the wire expressed in revolutions of the rheostat. 



The great increase of the resistance to more than four times its original 

 value, and its steady progress at such high temperatures, are very remarkable. 



The pjTometer was then placed in vacuo for a purpose that shall be subse- 

 quently noticed. As I feared that the sudden change of temperature might 

 fracture a glass receiver, a box of strong copper was used, the lid of which was 



VOL. XXII. c 



