358 Prof. Draper 071 the production of Light hy Heat. 



In fifT. 7, a i represents the slip of platinum, the upper end 

 of which is soldered to a stout and short copper pin a, firmly 

 sunk in a block of wood c, which is immoveably fastened on 

 the basis d d of the instrument. A cavity e, half an inch in 

 diameter, is sunk in the block c, and into this cavity the pin 

 a projects; so that when the cavity is filled with mercury, 

 a voltaic current may be passed through the pin and down 

 the platinum. ' 



Fig. 7. 



The other extremity of the platinum h is fastened to a de- 

 licate lever bf which plays on an axis at ^^j the axis working 

 in brass holes supported on a block Jt. Immediately beneath 

 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 71, weighing about 100 grains. 

 The further end of the index plays over a graduated ivory 

 scale p JK which is supported on a block §-, 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 im- 

 mediately 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 c^ the platinum instantly be- 

 comes cold, and pulls the lever to the zero point. 



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



