A NEW ELECTROSCOPE DESCRIBED. 215 



in any given case, there is a narrow graduated ring of cardboard or ivory, rr\ placed 

 behind it, the divisions being distinctly legible through sight- slits cut in the reeds : 

 the graduated circle is supported on a transverse rod of glass, by the intervention of 

 wood caps, and is sustained by means of the brass tube «, in which the glass rod is 

 fixed. The whole is insulated on a long rod of glass. A, by means of wood caps ter- 

 minating in spherical ends. In this arrangement, as is evident, the index diverges 

 from the fixed arms whenever an electrical charge is communicated to the ball h, as 

 in fig. 10. This instrument is occasionally placed out of the vertical position at any 

 required angle, by means of a joint at w, and all the insulating portions are carefully 

 varnished with a solution of shell lac in alcohol. 



8. Fig. 2. B. represents an electrometer which measures directly the attractive force 

 of an electrified body in terms of a known standard of weight estimated in degrees on 

 the graduated arc x y. An insulated conductor, /, is fixed on a varnished rod of 

 glass, f ^, sustained by the intervention of a wood ball on the extremity of a micro- 

 meter screw, s : by aid of the screw the whole may be raised or depressed, through 

 given intervals, to within the one hundredth of an inch of any required point. A 

 moveable and similar conductor, m, made of light wood, hollowed and gilded, is sus- 

 pended immediately over the former from the periphery of a small brass wheel W by 

 means of a fine silver thread attached near its vertical arm, and passing from thence 

 over its grooved circumference, as shown in fig. 3. The conductor m is counterpoised 

 by a short cylinder of wood, j) n, figs. 2, 3, suspended in a similar manner from the 

 opposite side of the wheel, by means of a silk thread : this counterpoise is partly im- 

 mersed in water contained in the glass vessel n, fig. 1. 



The extremities of the axis of the wheel W, figs. 2, 3, are turned to extremely fine 

 pivots, and rest on two large friction wheels, after the manner represented in fig. 4 *, 

 by which great freedom of motion is obtained. There is a fine index of light straw, 

 W c, attached to the extremity of a small steel needle inserted diametrically through 

 the circumference, which indicates on the graduated arc x y the force exerted be- 

 tween the conductors m /: the weight of this index is accurately poised by a small 

 globule of brass, t, fig. 3, moveable on a screw, cut in the opposite arm of the steel 

 needle carrying the index. 



The centre of the wheel W is accurately placed in the centre of the arc x y, which, 

 with its radii of support, is made of varnished wood, the graduated scale being of 

 cardboard or ivory. The arc is the sixth part of a circle ; it is divided into 120 equal 

 parts, sixty in the direction c x, and sixty in direction c ?/, the centre C being 

 marked zero. 



* I resorted to this method of employing friction rollers, as being more efficient than that in which the axis 

 is allowed to rest in the angles formed between the peripheries of four smaller wheels. In this case it rolls 

 fairly on a large circumference, and is prevented from passing off it on either side by the check wheels, either 

 of which, when acted on, opposes little or no resistance to motion. When this machine is equipoised with 500 

 grains, less than the -sVth of a grain will set the whole in motion. ^ 



2 f2 



