490 REPORT— 18G7. 



paper of 1867, "On Atmospheric Electricity" (Philosophical 

 Transactions). 

 II. Symmetrical electrometers. 



Bohuenberger's electrometer. 



Divided-ring electrometers. 

 III. Attracted disk electrometers. 



Absolute electrometer. 



Long-range electrometer. 



Portable electrometer. 



Spring-standard electrometer. 

 § 4. Class I. is sufficiently illustrated by the examples referred to ; and it 

 is not necessary to exjjlain any of these instruments minutely at present, as 

 they are, for the present at all events, superseded by the di^dded-ring elec- 

 trometer and electrometers of the third class. 



There are at present only two known species of the second class ; but it is 

 intended to include all electrometers in ■which a symmetrical field of electric 

 force is constituted by two symmetrical fixed conductors at different electric 

 potentials, and in which the indication of the force is produced by means of 

 an electrified body moveable symmetrically in either direction from a middle 

 position in this field. This definition is obviously fulfilled by Eohnenberger's 

 well-known instrument*. 



§ 5. My first published description of a divided- ring electrometer is to be 

 found in the Memoirs of the Eomau Academy of Sciences f about 1856 ; but 

 since that time I have made great improvements in the instrument — first, by 

 applying a light mirror to indicate deflections of the moving body ; next, by 

 substituting for two half rings four quadrants, and consequently for an 

 electrified body projecting on one side only of the axis, an electrified body 

 projecting symmetrically on the two sides, and moveable round an axis ; and 

 lastly, by various mechanical improvements and by the addition of a simple 

 gauge to test the electrification of the moveable body, and a replenisher to 

 raise this electrification to any desired degree. 



§ 6. In the accompanying drawings, Plate V. fig. 1 represents the front 

 elevation of the instrument, of which the chief bulk consists of a jar of white 

 glass (flint) supported on three legs by a brass mounting, cemented round the 

 outside of its mouth, which is closed by a flat cover of stout sheet-brass, and 

 a lantern-shaped cover standing over a wide aperture in its centre. For 

 brevity, in what foUows these three parts will be called the jar, the main 

 cover, and the lantern. 



Pig. 5 represents the quadrants as seen from above ; they are seen in ele- 

 vation at a and h, fig. 1, and in section at c and d, fig. 2. They consist of four 

 quarters of a flat circular box of brass, with circular apertures in the centres 

 of its top and bottom. Their position in the iustnimeut is shown in figs. 

 1, 2, & 6. Each of the four quadrants is supported on a glass stem passing 

 downwards through a slot in the main cover of the jar, from a brass mount- 

 ing on the outside of it, and admits of being drawn outwards for a space 

 of about f of an inch (1 centim.) from the positions they occupj^ when the 

 instrument is in use, which are approximately those shown in the drawings. 

 Three of them are secured in their proper positions by nuts (c, e, e) on the out- 

 side of the chief flat lid of the jar shown in fig. 4. The upper end of the stem, 

 carrying the fourth, is attached to a brass piece (/) resting on three short legs 



* A single gold leaf hanging between the upper ends of two equal and similar drv piles 

 standing vertically on a horizontal plate of metal, one with its positive and the other with 

 its negative pole up. t Accademia Pontificia dei Nuovi Lincei. 



