Prof. Draper on the 'produclion of lAght by Heat. 359 



point b, where it is fastened to the index, the movements take 

 phice vvitli such promptitude and precision as to leave nothing 

 to be desired. When the heat has been very high and long 

 continued, the limit of elasticity of the platinum is some- 

 what overpassed, and it suffers a slight permanent extension. 

 But as the ivory scale p p can slide up and down a little, the 

 index is readily re-adjusted to the zero point. 



The temperature of the platinum depends entirely on the 

 force^of the current passed through it. By intervening coils 

 of brass wire of lengths adjusted beforehand, so as to resist 

 the current to a given extent, any desired temperature may 

 be reached. I found it convenient to intervene in the course 

 of the current one of Prof. Wheatstone's rheostats, so as to 

 be able to bring the index with precision to any degree, not- 

 withstanding slight changes in the force of the voltaic bat- 

 tery. 



The following are the dimensions and measures of the in- 

 strument I have used: — Length of the platinum strip, 1*35 

 inch; length of the part actually ignited, 1"14 inch; width of 

 ditto, j'^th of an inch ; length of the index from its centre of 

 motion to the scale, 7'19 inches; distance of the centre of 

 motion of index from the insertion of the platinum at the point 

 b, -22 incli ; multiplying effect of the index, 32'68 times; 

 length of each division on the ivory scale, '021 inch. From 

 this it would appear, by a simple calculation, using the coeffi- 

 cient of dilatation of platinum given by Dulong and Petit, 

 that each of the divisions here used is equal to 114'5 Fahren- 

 heit degrees. For the sake of perspicuity 1 have generally 

 taken them at 115^. 



The Grove's battery I have employed has platinum plates 

 tlu'ee inches long and three quarters wide ; the zinc cylinders 

 are two inches and a half in diameter, three high, and one- 

 third thick. As used in these experiments, it could maintain 

 a current nearly uniform for an hour. I commonly employed 

 four pairs. 



Among writers on optics, it has been a desideratum to ob- 

 tain an artificial light of standard brilliancy. The preceding 

 experiments furnish an easy means of supplying that want, 

 and give us what might be termed a " unit-lamp." A sur- 

 face of platinum of standard dimensions, raised to a standard 

 temperature by a voltaic ciuMcnt, will ;dways emit a constant 

 light. A strip of that metal, one inch long and „',jth of an 

 inch wide, connccteil with u lever by wiiich its expansion 



