MI'CROTASIM'ETER (from Gk. turcp6s, mi kros, small + tasis, extension + Airport, met ron, measure). An instrument invented by Thomas A. Edison in 1877 for the purpose of measuring very minute variations of teurpe•a ture or moisture. The action of the apparatus depends on the effect which the pressure of an expanding rod has upon the electrical resistance of a piece of Carbon placed in the circuit of a galvanic battery. A rod of vulcanite is used as the expanding element when it is desired to em ploy the instrument to ascertain slight variations in the beat vibrations coining from any object, as the sun, or a gas or electric light. This rod is adjusted in a strong frame kept at an equable temperature. so that no expansions or contrac tions shall exert any influence except those which take place in the vulcanite roil itself. In the chamber which receives one end of this rod or plate there is placed under a follower or slide a piece of carbon, which becomes compressed with great, force upon the expansion of the vulcanite rod. If radiant heat is to be measured, a large funnel is placed in front of the apparatus to gather the rays and throw them upon the rod or plate. When the rays increase in intensity
the rod expands, compresses the button, and changes its resistance, the variation of which is indicated by a galvanometer. The instrument was used to ascertain the variations in the radiation from the sun during the solar eclipse of July 28. 1878. It may also be used to note the variations taking place on a day when clouds are passing across the sun's disk, or when the transmission of its rays is affected by increase or decrease of moisture. It may be used as a delicate hygrometer by substituting in place of the vulcanite rod a body containing gelatin, which expands under the influence of moisture. The chief disadvantage possessed by this instru ment is that the carbon does not regain its orig inal resistance after the pressure is removed. Modern miero-radiometers and bolometers are, in addition, far more sensitive and are usually em ployed for the measurement of radiation.