26 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 



the platinum 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, p p, supported on a block, 

 q / the scale 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 let there 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 ex- 

 pands, the weight n lightly stretching it. The index/ 

 moves promptly over the scale, indicating the amount of 

 expansion, and therefore the degree of heat. If the wire 

 N be removed 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 #, where it is fastened to the index, the 

 movements take place with such promptitude and pre- 

 cision 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 somewhat 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 readjusted to the zero point. 



The temperature of the platinum depends entirely on 

 the force of the current passed through it. By inter- 

 vening coils of brass wire of lengths adjusted before- 

 hand, so as to resist the current to a given extent, any 

 desired temperature may be reached. I found it con- 

 venient to intervene in the course of the current a 

 rheostat, so as to be able to bring the index with pre- 



