Page:Popular Science Monthly Volume 14.djvu/151

Rh —Fig. 32 shows in perspective the latest form of the Edison microtasimeter, or measurer of infinitesimal pressure.

The value of the instrument lies in its ability to detect small variations of temperature. This is accomplished indirectly. The change of temperature causes expansion or contraction of a rod of vulcanite, which changes the resistance of an electric circuit by varying the pressure it exerts upon a carbon-button included in the circuit. During the total eclipse of the sun, July 29, 1878, it successfully demonstrated the existence of heat in the corona. It is also of service in ascertaining the relative expansion of substances due to a rise of temperature.

In Fig. 33 the important parts are represented in section, affording an insight into its construction and mode of operation.

The substance whose expansion is to be measured is shown at A. It is firmly clamped at B, its lower end fitting into a slot in the metal

plate, M, which rests upon the carbon-button. The latter is in an electric circuit, which includes also a delicate galvanometer. Any variation