Page:Scientific Memoirs, Vol. 1 (1837).djvu/19

Rh obtained by means of the thermomultiplier, and a source whose radiation is much weakened by distance.

The apparatus is disposed in the following manner. A thermoelectric pile of thirty pairs is closed at one end and enveloped, at the other, in a small tube blackened inside to prevent reflection. At a certain distance there is placed a large metallic diaphragm, with an aperture at the centre equal to the section of the pile. On the other side, in the same line, there is a lighted lamp, which is brought more or less close, until the needle which serves as the index of the galvanometer, marks an elevation of 30°. The radiation is afterwards intercepted by a screen of polished metal placed between the lamp and the diaphragm, and the needle returns to zero. Then there is placed on the other side of the diaphragm a stand, with a plate of glass fixed on it, and the whole apparatus is moved gently until it is brought midway between the pile and the calorific source.

This being done, the opake screen is removed; the rays passing through the glass fall on the pile, and immediately cause the galvanometer to move. In 5s or 6s it is driven through an arc of nearly 21°·5, but it afterwards returns nearly to zero, oscillates in an arc of greater or less extent, and at last settles definitively at 21°. This last deviation decidedly marks the whole effect; for it is useless to continue the experiment for 15s or 20s. There is no longer any perceptible movement.

The time which the needle takes to attain its position of steady equilibrium is a minute and a half. When the experiment is repeated