[ 8 ] 



II. Description of an Elcclrical Plate Maehine, the Plate 

 mounted horizontaUy, and so as to show both negative and 

 positive Electricity, Illustrated by Engravings. By Robert 

 Hare, M.D. Professor of Chemistry in the University of 

 Pen nsylvwiia * . 



npHE power of electrical yAate machines has been generally 

 -■■ admitted to be greater than that of machines with cylin- 

 ders. The objection to the former has been, the difficulty of in- 

 sulating the cushions, so as to display the negative electricity. 

 Excepting the plate machine contrived by Van Marum, I have 

 read of none in which this difficulty has been surmounted. It 

 is still insisted upon, by respectable electricians, as if it had 

 not been sufficiently removecl by his contrivance. 



I presume, therefore, that a description of a plate machine, 

 by which both electricities may be shown, and which, after 

 two years' experience, I prefer on every account, may not be 

 unacceptable to the public f. 



My plate (thirty-four inches in diameter) is supported upon 

 an upright iron bar, about an inch in diameter, covered by a 

 very stout glass cylinder, four inches and a half in diameter, 

 and sixteen inches in height, open only at the base, through 

 which the bar is introduced, so as to form its axis. The sum- 

 mit of the bar is furnished with a block of wood, turned to fit 

 the cavity formed at the apex of the cylinder, and cemented 

 therein. The external apex of the cylinder is cemented into 

 a brass cap, which carries the plate. The glass cylinder is 

 liable to no strain ; it is only pressed where it is interposed 

 between the block of wood within and the brass cap without. 

 The remaining portion of the cylinder bears only its own 

 weight, while it effectually insulates the plate from the iron 

 axis. The brass cap is surmounted by a screw and flange ; 

 by means of which, a corresponding nut, and disks of cork, 

 the plate is fastened. A square table serves as a basis for the 

 whole. The iron axis, passing through the cover of the table, 

 is furnished with a wooden wheel of about twenty inches dia- 

 meter, and terminates below this wheel in a brass step, sup- 

 ported on a cross of wood, which ties the legs of the table dia- 

 gonally together. The wheel is grooved, and made to revolve 

 by a band, which proceeds from around a vertical wheel out- 

 side of the table. This external wheel has two handles : it 

 may of course be tunied by means either of one or both. It 

 is supported on two strips of w ood, which by means of screws 

 may be protruded lengthwise from cases, which confine them 



* Communicated hy the Author. f See Plate I. Fig. 3, 



from 



