XIII. Quadrant electrometers

Abstract

In 1886, on continuously charging up the needle of Sir William Thomson’s bifilar quadrant electrometer, No. 5, made by Messrs. White, of Glasgow, and in use at the laboratories at the Central Institution, it was noticed that the deflection of the needle, when the same P. D. (potential difference) was maintained between the quadrants, instead of steadily increasing, first increased and then diminished, so that both for a large charge on the needle as well as for a small, the sensibility of the instrument was small. A similar effect had been described by Dr. J. Hopkinson in the Proceedings of the Physical Society,’ vol. 7, Part I., for the previous year, and the explanation he gives of this curious result is that if the aluminium needle be below the centre of the quadrants the downward attraction of the needle, which increases with the square of the needle’s charge, increases the pull on the bifilar suspension, and so for high charges more than compensates for the increased deflecting couple due to electrical action. On raising, however, the needle of our electrometer much above the centre of the quadrants, the anomalous variation of sensibility of the instrument, with increase of charge in the needle, did not disappear, and even when the needle was raised so that it was very close to the top of the quadrants, and when, if Dr. Hopkinson’s explanation were correct, the sensibility (or deflection correspond­ing with a given P. D. between the quadrants) ought to have been very great for a large charge on the needle, it was, on the contrary, found to be small. As the needle had been somewhat damaged while the instrument was in the possession of the late Mr. Cromwell Yarley, before it was so kindly made over to the Central Institution by his son, Mr. Cromwell Yarley, Junior, together with other apparatus belonging to his father, a new needle, platinum wire, and weight, were obtained from Messrs. White. On suspending the new needle with the fine platinum wire and weight, as received from Messrs. White, the needle was found to have two modes of vibration, one the ordinary slow one, when the platinum weight turned in the acid at the bottom of the Leyden jar, and the other a very quick one, due to the twisting of the wire itself without the weight moving. This quick vibration was removed by replacing the fine platinum wire supplied by Messrs. White with a somewhat thicker one. The quadrants were adjusted for symmetry, the silk fibres tightened so as to have equal tension, as shown by the sensibility for a given potential of the needle, being a minimum, and the electrometer again tried, but it was still found that when the quadrants were close to one another, and when, therefore, as the needle was best shielded from external action, the standard formula for the electrometer might be expected to be most nearly fulfilled, the sensibility as before, first increased as the charge in the needle was increased, and then steadily diminished for further increase of charge on the needle. It was now, however, observed that, if the distance separating the quadrants was increased to inch, the sensibility tended to a limit for a large charge instead of first increasing and then diminishing.