MICROTASI3IETER, an instnirnent invented by Mr. Thomas A. Edison for the purpose of measuring. very minute variations of pressure caused by the expansion or contraction of any given body, from whatever causes, heat, moisture, etc. A part of the apparatus is constructed upon the principle of the pyrometer, and when the expan sion is caused by moisture, upon that of some forms of hygrometer. But the novel and unique part of the invention consists in the effect which the pressure of the expandinr• rod has upon the electric resistance of a piece of carbon placed in the circuit of a gals vanic battery. A rod of vulcanite is used as the expanding element when it is desired to use the instrument to aseertatn slight variations in the heat vibrations coming from any object, as the Still, or a gas, or electric light. This rod is adjusted in a strong frame kept at an equable temperature, so tliat no expansions or contractions shall exert any influence, except those which take place in the vulcanite rod itself. Li 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 contpressed 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 conducting capacity, which at every momeut is indicated by- a galvanom eter. The instrument has beeu used successfully to ascertain the variations in the radiation from the sun during an eclipse. It may ago 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 his rays differs from increase or decrease of moisture. It may be used a.s a delicate hygrome-ter by substituting in place of the vulcanite rod a body containing g-elatine, which expands under the influence of moisture.