ELECTRICITY. 



515 



Dr Hamcl's 



PLATE 

 CCALVIII 



Fig. 7. 



Fig. 8. 



against the woollen cylinder, and produced electricity 

 Klectricity. j n abundance. Beneath the drum he placed a pan con- 

 T"""' taining burning charcoal, in order to keep the appara- 

 tus dry. Bohnenberger has constructed several ma- 

 chines upon this principle. Their principal recom- 

 mendation is their cheapness and simplicity. 



8. Dr HameFs Electrifying Machine. 



This machine, of which we have been favoured with 

 an account by Dr Hamel of St Petersburg, is an im- 

 provement upon that of Lichtenberg. The cylinder 

 of this machine is represented in section in Fig. 7. 

 where A is its axis, and BC the cylinder, which is co- 

 vered wi'h wax taffetas. One end of the taffetas is fix- 

 ed by silk strings, in two grooves on the circumference 

 of the board B. The other end is lined with leather, 

 along the edge of which are made a number of small 

 holes, through which a silk string is conducted, and led 

 round as many wooden pins, placed on the external sur- 

 face of the board C, at some distance from the circum- 

 ference. By this means the taffetas is stretched, and it 

 is also forced into the grooves c. c. made upon the cy- 

 linder be means of a silk string tied round the peri- 

 phery of the board C'. When the taffetas becomes loose 

 atler long use, this string is taken off till the taffetas is 

 a<;ain brought to a proper degree of tension. The cy- 

 linder, when mounted on its frame, and covered with 

 a catskin, is shewn in Fig. 8. The catskin reaches from 

 F to (J, and has at F three silk strings, by which it is 

 hooked on as many wooden hooks, H, H, H, fixed to 

 the principal board of the machine. It is merely laid 

 round the taffetas cylinder, and the attraction pro- 

 duced by the electricity, excited during the motion of 

 the cylinder, is more than sufficient to keep it in every 

 part perfectly close to the tafVetn*. This is mi advan- 

 tage which cannot be obtained in arty machine, where 

 the cylinder or plate presses against :i fixed cushion. 

 The contact between the rubber and the cylinder N 

 thus rendered totally independent either of the form 

 of the cushion, or the true centering of the cylinder. 

 The hairy side of the catskin is placed in contact with 

 tin- taffetas, and its other side may be covered with silk 

 or any other stuff. In order to prevent the cylinder 

 from turning in the wrong direction, a wooden ratchet 

 wheel K is fixed to the axis, and its hook to the frcnie 

 of the machine. The conductor M receives the electri- 

 city from the cylinder merely by one single point. As 

 tskin should always be very dry, it would l>e con- 

 venient to have two, one of which ought to be drying at 

 the fire when the other is in use. Dr Hamel has oli- 

 ervi"d that these machines always act most powerfully 

 in n dry cold winter day, when the temperature of the 

 extern.-! air was 3O or more degrees below the zero of 

 Fahrenheit's scale, whatever was the temperature of the 

 room in which the machine was placed. 



CHAP. II. 



Description 

 of the elcc- 

 troplionu 

 f Volta. 



PlATE 



i.VIII. 

 tig. 9. 



Description of the Electrophorus of Volta. 



As the electrophorus of Volta is obviously an electri- 

 fying machine of a particular form, the description of 

 it belongs to the present Chapter. This ingenious in- 

 strument was invented by M. Volta, professor of natu- 

 ral philosophy at Como, and now at Padua, about tin- 

 year 1774. It is represented in Plate CCXLVIII. Fig. 

 ". where A is a circular metallic plate, or a plate of 

 wood covered with tinfoil, which has a glass handle FED 

 screwed into a brass or wooden nut D. The edge of 



this plate must be pretty thick, and well smoothed and 

 rounded off. The lower plate B consists of a resinous 

 cake placed upon another metallic plate, and formed by 

 pouring the resinous compound when fluid upon a 

 metallic plate with a circular rim. This resinous plate 

 is formed by melting together equal parts of shell lac, 

 rosin, and Venice turpentine ; and if the plate is re- 

 quired to be formed separate from the lower conductor, 

 this composition should be poured while hot upon a 

 marble table, from which it can be readily separated 

 when cold. The three plates which compose the elec- 

 trophorus may be distinguished by the letters A, B, C, 

 A being the upper conductor, B the resinous plate, and 

 C the lower conductor. The leading phenomena exhi- 

 bited by this instrument are given in the following ex- 

 periments : 



Exp. 1. If the resinous plate has been formed by 

 being melted upon the lower conductor in a state of 

 insulation, it will be found to possess negative electri- 

 city, and a spark may be drawn from any part of the 

 compound plate BC, particularly from the lower con- 

 ductor. This electricity, however, gradually dissipates. 



Exj). 2. Rub the resinous plate with a piece of flan- 

 nel, or fur, or, what is still better, whip it with a fox';, 

 tail, or a piece of catskin, and having placed the upper 

 conductor A upon the resinous plate B, raise it again 

 by its insulating handle, and the upper conductor will 

 exhibit no signs, or very faint signs of electricity. 



E.rp. 3. Replace the upper conductor upon the resi- 

 nous plate B, and touch it when in this situation with 

 the finger, or with any other uninsulated conducting 

 substance. Let it then be raised by the handle, and it 

 will not only exhibit indications of positive electricity, 

 but will afford a sharp spark. Let the plate A be again 

 placed upon B, and again raised, and it will afford ano- 

 ther spark, and this process may be repeated for a con- 

 siderable time, till no spark is obtained. This exhaus- 

 tion may be effected at once; for if when we touch the 

 I plate A with the finger, we at the same time touch 

 the lower conductor with the thumb, a sensible shock 

 will be felt in the finger and thumb, and they will no 

 loiifji-r exhibit any electricity. 



/.'/). <k Let the upper conductor A be now placed 

 upon the excited plate I!, by means of its glass handle, 

 nnd then bring the knob of a I.eyden jar in contact with 

 it. The leaves of an electrometer when touched with 

 the knob of the jar, will then diverge with negative 

 electricity. Let the upper conductor be now raised, 

 and the knob of the jar again presented to it, and a 

 spurk will pass between them. If the knob of the jar 

 is now brought into contact with the negatively electri- 

 fied electrometer, its divergence will be instantly de- 

 stroyed. This effect is obviously produced by an equal 

 qu.-mtitv of positive electricity ; for if the positive elec- 

 tricity had exceeded the negative electricity, the leaves 

 would have again separated with the excess, and re- 

 mained slightly positive; on the contrary, if the nega- 

 tive electricity had exceeded tile positive, the divergence 

 would not have been totally destroyed. This experi- 

 ment is by Mr Singer. 



Exp. 5. If the lower conductor C is now insulated, 

 it will exhibit negative electricity, when the resinous 

 plate 1? only is placed upon it, but when the upper con- 

 ductor A is placed upon B, the electricity of the lower 

 conductor C will change to positive. When the plate 

 A is again raised, C wdl become negative. Hence the 

 two conductors are always in opposite states of elec- 

 tricity. 



When the clectrophorus is well made, it may be em. 



PracticHl 

 Electricity. 



PLATE 

 CCXLVIII. 

 Fig. 9. 



Experi- 

 ments with 

 the elect ts 

 phorus. 



