Scientific memoirs : being experimental contributions to a knowledge of radiant energy / by John William Draper.

  • Draper, John William, 1811-1882.
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: Scientific memoirs : being experimental contributions to a knowledge of radiant energy / by John William Draper. Source: Wellcome Collection.

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    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, weighing about 100 grains. The further end of the index plays over a graduated ivory scale, 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 Avire N be removed out of its mercury cup 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 j^jt? 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-