Prof. Draper on the Production of Light by Heat. 401 



tinum strip, and in metallic communication with it, a straight 

 copper wire dips down into the mercury cup m ; on this wire 

 there is a metal ball n, weighing about 100 grains. The further 

 end of the index plays over a graduated ivory scale pp, which 

 is supported on a block q, and can be moved a little up and down, 

 so as to bring its zero to coincide with the index at common 

 temperatures. 



The action of the instrument is readily understood. In the 

 mercury cup e dip one of the wires N of a Grove's battery of 

 three or four pairs, the other wire P being dipped into the cup m. 

 the current passes through the platinum, which immediately 

 expands, the weight n lightly stretching it. The index /moves 

 promptly over the scale, indicating the amount of expansion, 

 and therefore the degree of heat. Remove the wire N out of its 

 mercury cup e, the platinum instantly becomes cold, and pulls 

 the lever to the zero point. 



When the platinum is thin, so as to be quite flexible at the 

 point 6, where it is fastened to the index, the movements take 

 place with such promptitude and precision as to leave nothing to 

 he desired. When the heat has been very high and long con- 

 tinued, the limit of elasticity of the platinum is somewhat over- 

 passed, and it suffers a slight permanent extension. But as the 

 lv ory ^ scale p p can slide up and down a little, the index is readily 

 ^adjusted to the zero point. 



The temperature of the platinum depends entirely on the force 

 °f the current passed through it. By intervening coils of brass 

 w tfe of lengths adjusted beforehand, so as to resist the current to 

 <J given extent, any desired temperature may be reached. I 

 tound it convenient to intervene in the course of the current one 

 °f Prof. Wheatstone's rheostats, so as to be able to bring the 

 index with precision to any degree, notwithstanding slight changes 

 ln l he force of the voltaic battery. 



The following are the dimensions and measures of the instru- 

 ment I have used: — Length of the platinum strip, 135 inch; 

 kngthof the part actually ignited, 1-14 inch; width of ditto, 

 iVth 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 

 Wdex from the insertion of the platinum at the point 6, 22 inch ; 

 multiplying effect of the index, 32-68 times ; length of each di- 

 J^sion on the ivory scale, -021 inch. From this it would appear, 

 b y a simple calculation, using the coefficient of dilatation of pla- 

 tmum given by Dulong and Petit, that each of the divisions here 

 Used is equal to 114-5 Fahrenheit degrees. For the sake of per- 

 spicuity I have generally taken them at 115°. 



The Grove's battery I have employed has platinum plates 

 three inches long and three-quarters wide ; the zinc cylinders are 

 tw o inches and a half in diameter, three high, and one-third 



Sscowd Series, Vol. IV, No. 12.— Nov., 1847. 51 



