The effect of glass covers in magnetic instruments

Abstract

Some time in the winter 1896–7, while determining index corrections of the compass declinometers belonging to the Coast and Geodetic Survey, Assistant G. R. Putnam observed motions of the magnetic needles of such nature as to leave no doubt that the glass covers of these instruments carried strong electrostatic charges, caused by atmospheric conditions, which at the time were especially favorable, the weather being dry and cold. In two of the declinometers, in which the glass covers were afterwards replaced by brass plates, with only small pieces of glass covering the openings through which to observe the points of the needles, no erratic movements were noticed, and repeated determinations of their index corrections, agreed with each other within the error of observation; while, before, nearly all observers using these instruments reported occasional discrepancies in their results, which could not be accounted for.