ELECTROMETERS. 2-11 



only form which can be used for obtaining with the aid of 

 photography a continuous record. At Kew Observatory a 

 quadrant electrometer is now in action ; and it is very much 

 to be desired that at every meteorological observatory atmo- 

 spheric electricity should be made a subject of continuous 

 observation. It seems strange that it has not already re- 

 ceived more attention ; or rather that but few observatories 

 have hitherto given the subject any attention whatever. 

 [Some of the photographic traces obtained at Kew with an 

 older form of reflecting divided ring electrometer, exhibited 

 in the Loan Collection, were shown and described.] 



I have just one other class of electrometers to refer to. 

 Electrometers of this class are called " attracted-disc elec- 

 trometers." The first instruments of this kind were made 

 by Sir W. Snow Harris, but they were in many ways very 

 imperfect, and, as I had occasion to remark yesterday, his 

 interpretation of the results which he obtained led him into 

 the error of disputing the truth of the laws of Coulomb. 



[The diagram (Fig. 8) shows an electrometer of Snow 

 Harris. As exhibited, it is connected with a Leyden jar, j, 

 which is to be tested, while the Leyden jar is being charged 

 from an electric machine through the well-known " unit jar " 

 L.] In this electrometer the attraction between two plates 

 a and d, one of which, a, is insulated and electrified, is 

 determined by weighing, as in a common balance. The 

 plate a, being electrified, attracts d, and the attraction is 

 counterbalanced and weighed by putting weights into the 

 pan P of the balance. Now if we have two circular elec- 

 trified plates attracting each other, the "lines of force" 

 between the plates being straight lines, and if we measure 

 the areas of the plates, the distance between them and the 

 force of attraction, it is easy to deduce the difference of 

 potentials between them in absolute measure. This was 

 practically what Snow Harris attempted. 



As I have said, however, his arrangement was very 

 imperfect. The movable plate 'd is in no way protected 

 from inductive influence, and of the nature or extent of that 

 influence there is no possibility of taking any account. 

 Moreover, the arrangement of the instrument is not such 

 as to enable us to consider the lines of force between the 

 attracting plates to be sufficiently nearly straight lines. 

 Sir William Thomson, in taking advantage, for an absolute 



