Page:Popular Science Monthly Volume 17.djvu/31

Rh also serve as a means of measuring such small differences of pressure, and thus furnish a comparison between the causes which produced them. The essential principle of the tasimeter is shown in Fig. 6.



A firm standard, A, holds at its upper end a screw which works against a follower, H, to which is attached the metal cup, I. At the base, between two platinum plates, a, a, is the carbon, C; the platinum plates are in a battery circuit provided with a galvanometer. Upon the upper platinum rests a metallic cup, D. Between the two cups, I and D, is placed a piece, E, of any material upon which experiment is to be made. The expansions and contractions of E cause changes of pressure upon the carbon, and thus changes of resistance in the electric circuit which are indicated by the galvanometer. The screw-head is turned until the initial pressure is sufficient to deflect the needle a few degrees. After the needle comes to rest, the slightest change of pressure will be indicated. The delicacy of the instrument depends largely upon the coefficient of expansion of the material used at E. With a piece of hard rubber, upon which the heat from the hand placed a few inches away is allowed to act, there is a deflection in the needle of a galvanometer which is insensible to the action of a thermopile facing a red-hot iron near at hand. When extreme delicacy is required, a Thomson's reflecting galvanometer is employed in a Wheatstone bridge in the way indicated in Fig. 7. The tasimeter is placed at i, and adjusted to a given resistance. The resistance at a, b, c, is made the same. The galvanometer is placed at G, and the minutest change of resistance at i is indicated at the galvanometer scale.

The instrument is of service for a variety of uses. It is an excellent device for detecting and measuring small and almost inappreciable quantities of heat. In the total eclipse of the sun in 1878, by the aid