280 Professw W, Thomson, [May iS, 



rally the earth, with the metal case of the instrument, and the other 

 with the insulated half ring. The needle being, let us suppose, nega- 

 tively electrified, will move towards or from the insulated half ring, 

 according as the potential of the conductor connected with this half 

 ring differs positively or negatively from that of the other conductor 

 (earth) connected with the case. The mirror turns accordingly in one 

 direction or the other through a small angle from its zero position, and 

 produces a corresponding motion in the image of the lamp on the 

 screen on which it is thrown. 



II. The common house electrometer. — This instrument consists of; 



(1) A thin flint-glass bell, coated outside and inside like a Leyden 

 phial, with the exception of the bottom inside, which contains a little 

 sulphuric acid. 



(2) A cylindrical metal case, enclosing the glas« jar, cemented to 

 it round its mouth outside, extending upwards about an inch and a 

 half above the mouth, and downwards to a metal base supporting the 

 whole instrument, and protecting the glass against the danger of 

 breakage. 



(3) A cover of plate glass, with a metal rim, closing the top of 

 the cylindrical case of the instrument. 



(4) A torsion head, after the manner of Coulomb's balance, sup-r 

 ported in the centre of the glass cover, and bearing a glass fibre which 

 hangs down through an aperture in its centre. 



(5) A light aluminium needle attached across the lower end of the 

 fibre (which is somewhat above the centre of the glass bell), and a stiff 

 platinum wire attached to it at right angles, and hanging down to near 

 the bottom of the jar. 



(6) A very light platinum wire, long enough to hang within one- 

 eighth of an inch or so of the bottom of the jar, and to dip in the 

 sulphuric acid. 



(7) A metal ring, attached to the inner coating of the jar, bearing 

 two plates in proper positions for repelling the two ends of the alumi- 

 nium needle when similarly electrified, and proper stops to limit the 

 angular motion of the needle to within about 45° from these plates. 



(8) A cage of fine brass wire, stretched on brass framework, sup- 

 ported from the main case above by two glass pillars, and partially 

 enclosing the two ends of the needle, and the repelling plates, from all 

 of wliich it is separated by clear spaces, of nowhere less than one-fourth 

 of an inch of air. 



(9) A charging electrode, attacRed to the ring (7), and projecting 

 over the mouth of the jar to the outside of the metal case (2), through a 

 wide aperture, which is commonly kept closed by a metal cap, leaving at 

 least one-quarter of an inch of air round the projecting end of the electrode. 



(10) An electrode attached to the cage (8), and projecting over the 

 mouth of the jar to the outside of the metal case (2), through the centre 

 of an aperture, about a quarter of an inch diameter. 



This instrument is adapted to measure differences of potential 



