1. electrometer, capillary An electrometer for measuringpotential difference by capillary action, which latter is affected by electrostatic excitement. A tube A contains mercury; its end drawn out to a fine aperture dips into a vessel B which contains dilute sulphuric acid with mercury under it, as shown. Wires running from the binding-posts a and b connect one with the mercury in A, the other with that in B. The upper end of the tube A connects with a thick rubber mercury reservoir T, and manometer H. The surface tension of the mercury-acid film at the lower end of the tube A keeps all in equilibrium. If now a potential difference is established between a and b, as by connecting a battery thereto, the surface tension is increased and the mercury rises in the tube B. By screwing down the compressing clamp E, the mercury is brought back to its original position. The microscope M is used to determine this position with accuracy. The change in reading of the manometer gives the relation of change of surface tension and therefore of potential. Each electrometer needs special graduation or calibration, but is exceedingly sensitive and accurate. It cannot be used for greater potential differences than .6 volt, but can measure .0006 volt. Its electrostatic capacity is so small that it can indicate rapid changes. Another form indicates potential difference by the movement of a drop of sulphuric acid in a horizontal glass tube, otherwise filled with mercury, and whose ends lead into two mercury cups or reservoirs. The pair of electrodes to be tested are connected to the mercury vessels. The drop moves towards the negative pole, and its movement for small potential differences (less than one volt) is proportional to the electro-motive force or potential difference.
Definitions of electrometer, capillary
