L5 



i; LECTUK IT V. 



\\itli ti.t- i;oii di-c, it vu necewiii : tin- fibre 



:iiul the hall .1 no.v slopped at :'.() a from the iron 



disc. He then instantly toucbod tin- iron disc with a 



*m:ill circle of p,i|KT of tin- - unc diameter, and having 



v. ithdniMit it. lie found that, i" order to make the_ hall 



ii stoj, it wa* nei. --.try to untwist the fibre, 



iucc tin- torsion to 





Now. in the lirst of these experiments, the copper ball 

 ' having, before its contact with the pith ball, repelled 

 1, '., tht- ball u to -'.s , while the micrometer marked 120 , the 



,.n was then 12ti 4-28 = I 4S. But after 

 it- ;t!i tlie pith ball, it repelled the ball a only 



t. 28 . while the micrometer indicated only 44 '.so that 

 tlie total forest of torsion, equal to the repulsive force of 

 the two balls. \va* only 44 4-28=.72. A minute, how- 

 ever, hail cl.qmd between the two observation-, and 

 :he di--ip,-ition was then T ' th per minute, the total 

 furcc <:f torsion must be corrected by ^th of 7'2 or 1^ 

 nc.irly. so tliat the corrected force of torsion will be 

 734, which diflcr.s only ^ J.th from H\ the half of 

 the former total force of tor.-ion which measured 

 the electric repulsion before tlie electricity of the cop- 

 r ball was communicated to the ball of elder : Since 

 tlie ili-t.-uu-e nf the balls, therefore, was in both cases 

 the same, and since the action is directly as the densi- 

 ties of the fluid, and inversely as the square of the 

 distance, it follows, that the pith ball has received exact- 

 ly one half of the electricity of the ball <f cnpjter, and 

 that the ball of copper has no more affinity or elective 

 attraction for the electric matter than the pith ball. 



In the second experiment, also, the electricity was 

 distributed equally between the iron disc and the disc 

 of paper, and by trying various other substances, and 

 repeating the experiments by means of a large torsion 

 balance with globes of five or six inches in diameter, 

 Coulomb always obtained the same result. 



Precautions J n repeating the preceding experiments, we must 

 *' take c.ire to allow the pith and the copper balls to re- 

 ine'the'tx-" m ain a short time in contact; for when one of the sub- 

 j^uienu. stances is an imperfect conductor, several seconds elapse 

 before it is able to deprive the other of one half of its 

 electricity. The time in which this distribution takes 

 place, depends not only on the conducting power of 

 t v iuo bodies, but on their relative extent, and on the 

 wanner in which the contact is effected. In bringing 

 into contact two circular discs, as in the second experi- 

 ment, we must take care that they touch one another 

 i-ymmetrically, or that the surface of the one is in the 

 same plane as the surface of the other. When this pre- 

 caution i* not taken, the electric matter will be distri- 

 buted unequally between the two discs. 



2. On the Superficial Distribution of Electricity. 



On the u- We have hitherto seen, that in the communication and 

 pcrticil distribution of the electric matter, every part of the 

 J V" n surface of an electrified body is charged with electricity; 

 we have now to inquire whether this electricity per- 

 vades the whole mass of the body, or is merely distri- 

 buted over its surface. The determination of this point 

 is owing likewise to Coulomb. As a more delicate 

 instrument was necessary for these experiments, he 

 made the following change in his apparatus: Having 

 formed a fibre of gum lac, about 10 or 12 lines long, 

 a'ld nearly as thick as a strong hair, one of its extremi- 

 ties was attached to the top of a small pin without a 

 head, impended to a silk fibre, such as comes from the 

 ilk worm ; tlie other extremity of the gum lac fibre 



city. 



hi ing fixed to .1 sii::ill . iide of tinsel about (wo lines _ . 



in diameter. When this electrometer i* suspended in 



a cylinder , - .-en.-ibility i- ,.,.,, -, ~ 



equal to a sixty ihoii.-andUi part of a grain is sufficient 

 to repel the ball and the me,!!,- to a distance of more 

 than ;)'.> degree*. I l.n '] communicated a small de- 

 f electricity to tin- circle of time!, dm! 



pehded it \- ],,. : ,jj; rt , 



currents of air. lie then ,1 cylinder of wood, 



four inches in ii d pierced with several hole-. 



four lines broad and four hues deep, and having p! 

 this cylinder upon an insulating stand, he gave it 

 ral electric sparks, either by means of a I.c\dcn J. 

 the metallic plate of an electrophorus. He next : . 

 lated, at the extremity of a small cylinder of guni ' 

 line in diameter, a circle of gilt paper a line ; .nd n half 

 in diameter, and with this apparatus he made the fol- 

 lowing experiments : 



47;. 1. The tinsel of the electrometer being electri- 

 fied, as we have already stated, he touched ilie si, 

 of the electrified wooden cylinder by the circle of gilt 

 paper, and upon presenting it to the cl.-ctrou-.eter, the 

 tinsel was repelled with force. 



r.ip. 2. He then introduced the circle of gilt paper 

 into one of the holes of the cylinder, so as to make it 

 touch the bottom of the hole, and UJHMI presenting it to 

 the electrometer no signs of electricity were exhibited. 



Now, in the first experiment, the small circle of gilt K Icrtririe . r 

 paper, which was only the eighteenth ]>.:rt of a line in ''' 

 thickness, became apart of the surface of the cylinder '',' 

 when it was made to touch it, and consequently received K 

 a quantity of electricity equal to that which \\.i- con- 

 tained in a part of the surface, equal to that of the 

 small circle. The small circle was not only charged 

 with a quantity of electricity perceptible to the small 

 electrometer, but capable of being exactly measured by 

 it. 



In the second experiment, on the contrary, when 

 the small circle was placed at the bottom of one of tlie 

 holes, four lines below the surface, and twenty lines 

 from the axis of the cylinder, and when it was" taken 

 out of the hole, so as not to touch its sides, it exhibited 

 no marks of electricity ; and hence it follows that tiiere 

 was no electric matter in the interior of the cylinder, 

 even at the small depth of four lines. On some occa- 

 sions, the circle of gilt paper exhibited signs of a weak 

 electricity, opposite to that of the cylinder, an effect 

 which Coulomb ascribes to a small degree of electricity 

 received by the gum lac, from its being within the at- 

 mosphere of the electrified cylinder. In order to prove 

 that this opposite electricity existed in the gum lac, and 

 not in the circle of gilt paper, he touched the gilt circle, 

 and was not able to destroy its electricity. The oppo- 

 site electricity of the gilt circle is, however, a! -.ays very 

 feeble when the gum lac is pure, and when the weather 

 is not very damp. 



S. On the Distribution of Electricity bet Keen t no con- 

 ducting Bodies in contact. 



In the experiments of Coulomb on this subject, he On the fa, 

 found it necessary to employ a torrien balance of a lar- t ,ii , 

 ger size than'that which is represented in Plate ('( XI. \ 

 and of a different form. The two balances which Le bL ' twein '' 

 used, are represented in Plate CCXLV. Fig. I, and '2. ^""j," 8 

 In Fig. 1, AB is a square box, formed by four plates contact 

 of glass, 2 feet long and 1.5 or 16 inches high, which i'i 

 must be placed upon a table, very dry, and coated with ' ' XI. V. 

 a non-conducting varnish. This square box is covered ^'8* ' * 



