. r ,20 



ELECTRICITY. 



Talc. 



rrtil was then coated with tinfoil ah* but the neck. Thetin- 

 ^- plate tbnned the inner coating and wire, and tin- tin- 

 *"Y"" 1'oil the outer coating, while the wax acted the part of 

 glass in the common jar. 



SECT. II. On Coaled Plat ft. 



W r. have already seen that electricity may be accu- 

 mulated as well in a coated plate as in a coated jar, and 

 we have represented a plate of this kind in Fig. 7- of 

 PlateCCXUlI l)r Bevis was tin- fiiM MMOB who 

 substituted coated plates of glass instead of jar*, and Dr 

 Franklin actually u.-cd a battery consisting of coated 

 plates of crown glass. As the accumulation of the 

 electric matter increases with the thickness ot' the glass, 

 Muscovy talc has lx.tn substituted in place of glass, and 

 it has the additional advantage of being !<-; liable to at- 

 tract moisture from the atmosphere. Instead of glass, 

 Beocaria employed a resinous plate consisting of equal 

 quantities ot pure colophony and powder of marble, and 

 having previously stuck a piece of tinfoil within five 

 inches of the edge of the table, he poured this composi- 

 tion, when melted, upon the table, and having spread 

 it to the thickness of a 10th of an inch with a hot iron, 

 he coated the tipper side with tinfoil, and found that it 

 had the power of accumulating a greater degree of elec- 

 tricity than an equal plate of glass, and was less liable 

 to be broken by a spontaneous discharge. 



SECT. III. On the Construction of Batteries. 



On the con. The electrical batten,' was invented in the year 17*6, 



(traction of foy Gralath, a German. l)r Franklin constructed one 



attend. Q f coate ,i plates of glass. Dr Priestley employed one of 



6t jars ; but the largest that have ever been made are 



those which Mr C'uthbertson made for the Teylerian 



Museum of Haarlem. 



The jars which compose a battery are formed and 

 coated in exactly the same manner as a single Leyden 

 phial. In Figs." 8 and 9, of Plate CCXLIII. we have 

 given a representation of two batteries, shewing two 

 different forms of the jar, and two different manners of 

 uniting the separate effects of each. 



Real mea- The real power of batteries ought not to be estimated 

 sure of the by the number of square feet of coated surface which they 

 rffccts of contain, but by the length of metallic wire which they 

 ** ttcnt5 - are capable of igniting or exploding. In the year I 774, 

 Mr Nairne constructed a battery, containing 50 square 

 feet of coated surface, and he found that it was capable 

 of igniting 45 inches of iron wire, T f rr tn P art *" an mcn 

 in diameter, which corresponds to about ^ths of an inch 

 of wire for -each square foot. The first battery con- 

 structed by Mr Cuthbertson for the Teylerian Museum, 

 contained 135 square feet of coated surface, and was 

 capable of igniting about 180 inches of the same kind 

 of wire, which is nearly one inch and ,V tns of wire for 

 each square foot, surpassing considerably the power of 

 Nairne's battery. When this battery was increased to 

 '2-25 square feet of coating, it ignited 300 inches of the 

 r line wire, which was also at the rate of one inch and 

 V^ths for each square foot. The large battery of 1 00 

 jar*, which Mr Cuthbertson afterwards completed for 

 the same mu-eum, contained 550 square feet, was ca- 

 pable of igniting (J55 inches of the same wire, which was 

 also at the rate of one inch and y^ths for every square 

 foot. Mr Cuthbertson afterwards made a number of 

 batteries of 15 jars each, and having about 17 square- 

 feet of coated surface. Calculating from Nairne's bat- 

 tery, these small ones ought to ignite- 1 j i inches of wire, 



and calculating from the Haarlem batteries, they ought fr '< 

 to ign . lu-, ; hut Mr (uthU-rtson loinid tin-in ' 



capable of igniting (it) inches of the same wire This " """"''"" 1 

 n-markable increase Mr ( uthlHTUon could not easily 

 expl.iin. The Haarlem batteries w ere made of Bohemian 

 glass, and the small batteries mentioned aho\ewrrc 

 made of Hint glass; but Mr Cuthbertson does not think 

 that the difference of power can be ascribed to this 

 cause. 



Mr Brooke had observed that a coated jar took a much Cuthbrn- 

 higher charge when it was dirty than when it was clean; wm ' b - 

 but he does not seem to have made any experiments to ^"^j'lf '5," 

 investigate this singular fact. In the year I7<)2, Mr p^crof 

 Cuthbertson happened accidentally to observe, that a b-iuettafc 

 jar when it is a little damp in the inside, (which it al- 

 ways is when fresh coated,) took a higher charge than 

 when it had been coated for some time and dry. He 

 then tried the power of a jar when it was damped by 

 breathing into it, and found that it was capable of igni- 

 ting 12 inches of wire, whereas it ignited only ;~> inches 

 when well dried. In March I7<)(>. in very dry weather, 

 Mr Cuthbertson charged a battery of 15 jars, and 17 

 square feet of coated surface, and found that it ignited 

 18 inches of wire; but after breathing into each jar 

 through a glass tube, and then charging the battery, he 

 WM Mtanttbed to find that it was then able to ignite 

 60 inches of the same wire. In order to examine thi* 

 subject more accurately, Mr C'uthbertson constructed 

 his compound electrometer, which will be described in 

 the next Chapter, and instituted a set of experiments 

 to determine in what degree the charging capacity of 

 coated jars is increased by breathing into them. The 

 result of these experiments was, that the force of batte- 

 ries did not increase with the quantity of coated surface, 

 but according to the law already given in page !(!). 

 Hence it resulted, that instead of the power of a battery 

 being increased * times, as he at first supposed, by 

 breathing into the jars, the increased effect was only 

 two-thirds. This increase, however, is still sufficiently 

 great to make us regard Mr Cuthbertson's discovery 

 of the effects of dampness, as one of the most valu- 

 able additions which has for a long time been made to 

 practical electricity. 



An effect, similar to that of breathing into the jar, 

 may be produced by placing a wet sponge within it, 

 or by slightly oiling its surface ; but as these me- 

 thods are not permanent, it has been recently proposed 

 to paste a slip of writing paper an inch broad on the 

 interior surface of the jar, so as to rise about half an 

 inch above the upper edge of the inner coating. 



SECT. IV. On Instruments for discharging Jars and Bat- 

 teries. 



A DESCRIPTION of the common discharging rod has al- On uistru. 

 ready been given in a former Section. Discharging menu for, 

 rods with only one branch, and having glass handles, didiargfaf 

 are shewn in Plata CCXLIX. Fig. 2. and are of great ["'"j^ 

 service in many experiments, for completing the elec- 

 trical circuit, by fastening a wire at the end a of the | 1 ' 1 ^'.'.^ 

 metallic branch all. A jar may be discharged, as repre- j.| ' g' 

 sented in Plate CCXI-IX. Fig. :J. by connecting with 

 the outside coating a bent arm lie, attached tearing dc, 8- " 

 and by suspending any conducting body c by a silk 

 thread between the knobs a and '. The body c will 

 oscillate between these bails till the jar is dischar- 

 ged. When the body e is shaped into the form of a 

 spider, this experiment has been called the elect rijicd 

 spider. 



