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needle, disc and jar

ELECTROMETER, an instrument for determining difference of electrostatic poten tial (or electric charge) between two charged conductors. The electroscope (9.v.) and Cou lomb's torsion balance are pnmitive forms. The attracted-disc electrometer was designed by Volta and developed by Snow-Harris. It con sists of a battery or Leyden jar to the wires of which is attached a horizontal disc. A bal ance is placed close by, having on one end of the beam a disc which is positioned a short dis tance above the disc connected with the jar, and on the other end of the beam a pan for containing small weights. In this manner the degree of force that will pull the two discs to gether is weighed in ounces or pounds. To secure a correct result it was found necessary to place a guard ring around the upper disc, and this later form was named the absolute electrometer. Through this mechanism it was demonstrated that the attraction between the discs at different distances varied as the square of the difference of potential. Lord Kelvin constructed an electrometer located in the in terior of a Leyden jar, and employing the tor sion of a wire to measure the difference of po tential. This invention was outclassed later

by his quadrant electrometer. This was de signed to measure the electrostatic charge by the attraction of quadrants of metal of lcnown attractive force on a very light aluminum needle. He hung a paddle-shaped aluminum foil needle, enclosed in a box, between the four insulated metal quadrants. Opposite quadrants were con nected by platinum wires. The difference of po tential when connected with one pair of quad rants or the other pair was made use of to de flect the needle, and a mirror and light being provided, the needle threw a spot of light on a scale. It was found necessary to provide the in strument with a “replenisher° to preserve the charge of the Leyden jar. The quadrant elec trometer was so much more delicate in its meas urements that it superseded the cruder ments, and being later improved by Dolezalek and others became a standard instrument. See