56 Profs. Ayrton and Perry and Dr. Sumpner. [June 4, 



needle, so that maximum sensibility, bordering on instability, is 

 obtained with this arrangement of the quadrants. 



After carrying out a large number of experiments, the cause of the 

 irregularity in the action of the Thomson quadrant electrometer, as 

 made by Messrs. White, began to dawn on us. The wire supporting 

 the aluminium needle, as well as the wire which connects the needle 

 with the sulphuric acid in the Ley den jar, is enclosed in a metallic 

 guard tube to screen the wire from external action. But, in order 

 that the needle may project outside the guard tube, openings are 

 made in its two sides. Hence the moment the needle is deflected 

 from its zero position, each half of the needle becomes unsymmetri- 

 cally placed relatively to the two metallic pieces which join the 

 tipper and lower half of the guard tube. Therefore, in spite of the 

 needle and the guard tube being always maintained at the same 

 potential, there is a repulsion between the charges on the two con- 

 necting pieces of the guard tube and the charges on the two halves of 

 the needle. And this repulsion has not only the defect of seriously 

 diminishing the sensibility of the quadrant electrometer as made by 

 Messrs. White, but causes the variation of sensibility of the electro- 

 meter with variation of the P.D. between the needle and the outer 

 coating of the Leyden jar to follow a far more complicated law than 

 that expressed by the conventional formula just given. 



To test this theory, that the peculiarities in the law of the quad- 

 rant electrometer are due to the electric action of the guard tube on 

 the needle in consequence of the special shape of the former, we 

 intensified and varied the want of symmetry of the guard tube by 

 attaching a piece of thin aluminium foil to it above and below the 

 needle, and experiments made on the law connecting the sensibility 

 of the electrometer with the potential of the needle showed that the 

 law could be much altered in character by a slight shift in the 

 position of the piece of aluminium foil. 



The paper then goes on to describe experiments connecting the 

 motion of the electrical zero with the potential of the needle, and 

 with the position of the adjustable quadrant. 



Guided by the results of a long course of experiments on the White 

 electrometer, we were led, with the assistance of Mr. Mather, to 

 construct an improved unifilar quadrant electrometer which is fully 

 described and illustrated in the paper. This improved electrometer 

 differs in numerous particulars from that made by Messrs. White. 

 The bifilar suspension is abandoned for reasons given in the paper, and 

 there is employed instead a new form of adjustable magnetic control, 

 so arranged that the needle is practically unaffected by outside mag- 

 netic disturbance. All the working parts are supported from the 

 base, so that on removing the glass shade, which serves as the Leyden 

 jar, all the parts can be got at and adjusted in position; all the 



