346 Prof. Draper o« the j)roduction of Light by Heat. 



3. To determine tlie relation between the brilliancy of the 

 light emitted by a shining body and its temperature. 



Here we shall find that the intensity of the light increases 

 far more rapidly than the temperatnre. For example, plati- 

 num at 2600° emits almost forty times as much light as it 

 does at 1900°. 



As I prefer to give a complete description of the apparatus 

 employed in these investigations after the general results are 

 stated, it is sufficient here to understand that the source of 

 light is in all instances a very thin strip of platinum 1*35 inch 

 long and g'^th of an inch wide, brought to the temperature 

 under investigation by a voltaic current. Platinum was se- 

 lected from its indisposition to oxidize, and its power of resist- 

 ing a high temperature without fusion. 



The slip of platinum, thus to be brought to different tem- 

 peratures by an electric current of the proper force, was 

 iastened at one end to an inflexible support, and at the other 

 was connected with a delicate lever-index, which enabled me 

 to determine its expansion, and thereby its temperature. For 

 this purpose I have used the coefficient of dilatation ot Dulong 

 and Petit. The temperatures here given are upon the hypo- 

 thesis of the invariability of that coefficient at all thermometric 

 degrees; they are therefore to some extent in error. 



By the aid of resisting wires of different lengths and a 

 rheostat, I was able to vary the force of the electric current in 

 the platinum, and thereby vary its temperature. My first 

 attempts were to discover the point at which the metal begins 

 to emit light. 



The platinum and the voltaic battery were placed in a dark 

 room, the temperature of which was 60^ ; and after I had re- 

 mained there a sufficient length of time to enable my eyes to 

 become sensible to feeble impressions of light, I caused the 

 current to pass, gradually increasing its force, until the pla- 

 tinum was visible. In several repetitions of this experiment 

 it was uniformly found that the index to which the platinum 

 was attached stood at the eighth division when this took place. 

 The metal had therefore dilated ^^-^ oi its length ; the eleva- 

 tion of its temperature was about 917°, which added to the 

 existing height of the thermometer, 60°, gives for the tempe- 

 rature of incandescence 977° F. 



To the correctness of the number.it may be objected, that 

 owing to the narrowness of the metallic strip it is not well 

 calculated to make an impression on the eye when the light it 

 emits is so feeble; nor can we take the dilatations given by 

 the index, as representing the uniform temperature of the 

 whole platinum, which must necessarily be colder near its 



