85 Defciipthii of an EL-ctrical Machine. 



their whole length ; the actions on the oppofite fides of the plate arc accurately tlic fame ; 

 and the play of the hinges gg, prevents the plate from being endangered by any ftrain in the 

 <lire£lion of its axis. It is certain that, before this adequate provifion was made to fecure 

 tliofc eirentiil requifites, it was impracticable to apply the cuftiions to a plate with the fsme 

 fafetyand effcil as to cylinders, which poflefs much ftrength from their figure. An inge- 

 nious workman will probably find little difficulty in conftrufting thcfe rubbers from this 

 dcfcription and drawing ; but the mod precife information refpc£ling every circumllance 

 and dimcnfions is to be found in the letters above quoted. 



The inner extremities of the cufbions are defended by the plates of gum-lac YY, which 

 cover the three fides or edges, and prevent their attriKfling the eledlric matter from the 

 ends of the receiving conduftor. 



1 hat part of the axis which moves between the collars is made of fleel. The middle of 

 the non-con(lu£ting part of the axis is a cylinder of walnut-tree wood aaaa, baked until its 

 infulating power is equal to that of glafs, and then foaked in amber varnifli, while the wood 

 (till remains hot. The two extremities of this cylinder,which are of a lefs diameter, are forced, 

 by ftrong blows, with a mallet, into the flout brafs caps b and c, in which they are retained 

 by three iron fcrews dd. The cylinder aa, and the brafs caps are covered with a layer of 

 gum-lac eeee, to preferve the infulating (tate of the wooden cylinder more perfedlly, and to 

 prevent the cap b from throwing flaflies to the rubbers. The bottom of the cap b is fcrewcd 

 home on the tapped extremity of the fteel axis b. The bafc of the cap c, wliieh is four inches 

 in diameter, terminates in an axis one inch thick, and two in length ; the extremity of 

 which is formed into a fcrew. The glafs plate is put on this projedting part, and fccured 

 in its place by a nut of box-wood, forced home by a key, applied in the holes ii. Two rings 

 of felt are applied on each fide of the glafs, to defend its furface from the eonta£l of the 

 wood and the metal ; and the central hole in the glafs, which is two inches in diameter, 

 contains a ring of box -wood, which prevents its immediate application to the axis. 



As it is neceflary that the axis G fhould be parallel to the axis of the plate, in order that 

 the conduflors FF may move parallel to the plate itfelf, the pillar M is rendered adjuftable 

 by three bearing fcrews RR at the bottom, which re-act aguinll the flrong central fcrew 

 T, and this is drawn downwards by its nut. The condu£lors FF are alfo adjuftable by the 

 fliding-pieces vv, and the binding-fci"ews ww, which alfo aiTord an adjuftment to bring the 

 axis of each fmall conductor parallel to the face of the glafs plate. A fimilar adjuftment may 

 be obferved at the extremities of the arc II. 



Fig. 4. reprefents a fedlion of the moving part of the branch II, one-half of its real fize. 

 A brafs plate out is fcrewed to the face of the capital K by three iron fcrews g. To this is 

 f.-rewed another ring JJ, v.'hich affords a groove for the moveable ring 77, into which the 

 arms II are fixed. This is accordingly applied in its place before the ring JJis fixed. 



The wooden part of the rubbers GG, Fig. 2, Plate IV, is covered with thin plates of iron, 

 excepting the furface neareft to the glafs. The intention of this i's to maintain a more per- 

 fect communication between the rubbed part of the cufliion and the earth or negative con- 

 ductor, as the cafe may be. 



The plates of gum-lac YY, are applied to ,the rubbers, each by means of a thin plate of 

 brafs, to wliich they are affixed by heat. There are two wires riveted in thcfe plates, which 

 are thruft into correfpondcnt holes in the wooden part of tlie cufliion. 



The 

 I 



