

ELECTRICITY. 



Abb* Nol- 

 bt'tnu- 



chinc. 



Light bodies were placed in the box below it, and when 

 put in motion, it was rubUd by the hand till the light 

 bodies were attracted by it. 



Mr Hawksbet-'s machine was a glass glol>e turned by 

 large multiplying wheel, which whirled it round with 

 great velocity. It had no conduct or. Dr Priestley has 

 given a drawing of it in his Hi \toiy of Elictricitij. 



The machine employed l>y the Abb Nollet, of which 

 a drawing is given by Dr Priestley, differs in nothing 

 but ita frame from that of Hawksbee. It was furnish. 

 ed, however, with a conductor, which was merely a bar 

 of iron, or a gun barrel, and sometimes a chain sus- 

 pended by a silken string coming from the ceiling of 

 the room. 



Mr Wilson's machine consisted of a cylinder, turned 

 with a multiplying wheel Two horizontal silk cords 

 were supported upon four vertical pillars of wood, and 

 the conductor, which was a bar of iron, rested in an in- 

 sulated state upon the silk cords. See a drawing of it 

 in Priestley's Hi /on/ of 'Electricity. 



Dr Watson's electrifying machine consisted of four 

 globes, supported in a vertical frame, and whirled round 

 by four different strings placed in four different grooves 

 on the surface of a large multiplying wfieel. His prime 

 conductor was a bar of iron suspended from the roof of 

 the room. A drawing of it may be seen in Priestley's 

 History of Electricity. 



SECT. II. On Electrifying Machines made nith circular 

 Discs of Plate Class. 



thielettri- THERE is one obvious disadvantage attending the 

 IneVmade co " structlon of electrifying machines made of globes or 

 with circu cylinders namely, the difficulty of cleaning their inte- 

 Ur dic of nor surface, in case of any moisture that may have been 

 flue glkw. accidentally introduced. In order to avoid this incon- 

 venience, large cylinders have sometimes been construct- 

 ed, so that the hand could be introduced at one end, 

 for the purpose of cleaning and drying the globe. In 

 order to avoid this evil, as well as to render the elec- 

 trifying machine more portable and less liable to injury, 

 a circular disc of plate glass was substituted in place 

 of the glass cylinder. The honour of this invention has 

 been claimed for our countryman Mr Ramsden, and also 

 by Dr Ingenhousz, physician to the Emperor of Ger- 

 many. Some persons have endeavoured to settle these 

 claims, by asserting that the idea was conceived by In- 

 genhousz, and executed by Ramsden, and there is every 

 reason to believe that this was the case. Dr Ingenhousz 

 proposed the use of a circular disc of glass in 1 763, and 

 mentioned the idea to Dr Franklin and several other 

 persons ; and among the additions at the end of the first 

 edition of Dr Priestley's History of Optics, published 

 in t7')~, we are informed that M r Ramsden had recent- 

 ly executed a plate glass machine, which turned ver- 

 tically, and was rubbed by four cushions, each 1 ^ inches 

 long, and placed at the opposite ends of the vertical 

 diameter. See PhiL Trans. 1779, P- 659. 



1. Description of the Plate Glass Machine. 



Flate gUw j\ small machine of this kind, chiefly intended for 



maclune ' medical purposes, is represented in Fig. 6. where AB 



TIATE is a circular disc of plate glass supported upon a hori- 



" zontal axis, the two extremities of which rest on two 



upright pillars E, F. The plate receives its rotatory 



motion from the simple winch W. The rubber consists 



of two pair of cushions, fixed at the opposite ends of a 



diameter of the circle, to pieces of mahogany m, m, so 



thin as to be considerably elastic. These cushions are Practical 

 *een more distinctly in the section of the machine given ' 

 in Fig. 8. where they are marked by the number- 1, ',', |. LXTE 

 3, 4. The cushions are kept tight again-t the p'-:te of , i \.].\'II 

 glass by mums of the .-crews si, si. In this m.-iehine Kig. 8. 

 the conductor CD has a particular form, for the pur: 

 of canreying the electricity tonjar.I p u-araspos- 



sible to the pillar F, in order to make the machine very 

 compact. When the machine is not used for medical 

 pnr|K>ses, the conductor has the form represented in 



l ). where AB is the plate of glass, W the handle F* ' 

 that turns it, and CDE the arched conductor, insula- 

 ted by a pillar of glass II, and carrying two sets of 

 points p n for receiving the electricity from the ma- 

 chine. To each pair of rubbers is attached a flap made 

 of oiled silk, one of them ascending and the other de- 

 scending to therows of points p,i>. These flaps are shaded 

 darker than tile glass. A discharging electrometer by 

 Lane, is represented in Fig. 6. as inserted in the top 

 of the jar. 



2. Cuthbertson' s Plate Glass Machine, with an Insulated 



stand. 



Mr Cuthbertson, to whom the science of electricity is Cudibert- 

 under great obligations, not only for many valuable ad- s""' 5 P late 

 ditions to, and improvements upon, electrical apparatus, B] 

 but for several excellent experiments, has brought the 

 electrical plate machine to a state of very great perfection. 

 As it is extremely difficult to insulate the rubbers of the 

 plate machine without giving it a clumsy appearance, 

 negative electricity may be obtained with equal case, 

 by insulating the whole machine, as in Fig. 8. where 

 it is supported by four glass pillars G, H, I, K. In Fl K- * 

 this construction, it is necessary to insulate the winch 

 \V hv a glass rod. Instead of making the whole stand 

 of one piece, as in Fig. 8. the machine may be fasten- 

 ed with clamps to a glass stool, of the same size and 

 shape as the stand of the machine. The conductor, 

 insulated by a rod of glass r, is shewn at CD. 



3. Culkbertson's Double Plate Machine, made for the 



Teylertan Museum at Haarlem. 



The double plate machine, as constructed by Mr CutMwrt- 

 Cuthbertson, differs from the preceding only in having son's double 

 a greater interval between the two upright pillars E, F, P^tc m - 

 for receiving two circular plates instead of one. The chine ' 

 two plates are fixed parallel to each other, on the same 

 axis, and are turned by the same handle. Two branch- 

 es of the conductor are introduced between the two 

 plates, and have rows of points on each side for carry- 

 ing off the electricity from the glass. Hence it is ne- 

 cessary to support the conductor by an insulated eland 

 separate from the machine, instead of placing it in a 

 dovetailed groove, as is done in Fig. 8. In the machine Teyieriw 

 made upon this principle by Mr Cuthbertson for the ma ihine. 

 Teylerian museum at Haarlem, each of the circular 

 plates -wasjivrfcctfae indies in tliameter, their distance 

 7^ inches, and the length of the rubbers 1 5^ inches. In 

 order to strengthen the plates, and to prevent any dissi- 

 pation of electricity along the axis, the two circular 

 plates were coated with a resinous substance to the dis- 

 tance of I6i inches from the centre. The prime con- 

 ductor consisted of several pieces, and was sustained by 

 three pillars of glass 57 inches high. It was by means of 

 this machine, which is celebrated in the history of elec- 

 tricity, that Van Marum made his experiments on the 

 fusion of metals, of which we have given a full account 



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