ELECTRICITY. 



451 



PLATE 

 :< XI.V. 



Descriptive with glass, so as to leave an opening for introducing 

 ' the globe a, placed at the end of a small cylinder ac of 

 gum lac, which is terminated upwards in a small cylin- 

 der of baked wood, coated with gum lac, and passing 

 perpendicularly through a hole in the piece of wood cd, 

 in which it is stopped by a screw c. The vertical tube 

 m n from 1-2 to 15 inches high, is made of glass, and is 

 Upported by a frame pqrs, carrying a semicircle /, 0, , 

 about four feet in diameter, having its centre coinci- 

 dent with the vertical line m n, and divided into two 

 quadrants of f)0 J each, beginning at 0. The suspend- 

 ing fibre m n carries the piece u r, to which is attached 

 a thread of gum lac n b, terminated in h by a small disk 

 of gilt paper. 



Pig. 2. Represents another balance of the same 

 kind, but still more simple. The four vertical and 

 square plates of glass, rise from a groove in a piece of 

 well baked wood. Instead of the semicircle /, 0, n; a 

 band of p.ipcr > ;; is pasted on one of the glass plates, 

 and on both sides from is divided into degree?, ac- 

 cording to the tangents of a circle, whose centre is the 

 suspending fibre. 



The micrometer m, at the top of both these balances, 

 does not differ materially from that which has been re- 

 presented in a former Figure. The head of it consists 

 of two circles, the lowest of which has a space of five 

 degrees divided into single degrees, while the other cir- 

 cle is divided into spaces of five degrees each. These 

 two circles are intended to adjust the needle to the 

 zero of the scale. The suspending fibre in both these 

 balances was a copper wire of the diameter No 12. in 

 commerce. 



In using the preceding apparatus, Coulomb employ - 

 mcthtxisof ed two methods for determining the manner in which 

 measuring the electric matter distributed itself between two bodies 

 tbediiuibu. j n contact. In ihcjirsl, he placed the electrified body 

 in the balance, after having electrified in the same man- 

 ner the small circle of gilt paper at the end of the needle. 

 The needle being repelled through a certain distance, 

 was then brought back by means of the micrometer to 

 any distance from the electrified body; and the angle of 

 torsion given by the micrometer, added to the distance 

 of the needle from the zero of the scale, measured 

 the repulsive force which the bodies exercised at that dis- 

 tance. The electrified body placed in the balance, 

 was then touched by the body with which it was to di- 

 vide its electricity ; and in untwisting the suspending fi- 

 bre, the needle was brought back to the same distance 

 from the body as formerly, and then the angle of tor- 

 sion added to the distance of the needle from zero, 

 measured the quantity of electricity which was left 

 in the body by the other, to which part of its electricity 

 was distributed. In this method, unless the weather is 

 very dry, it is necessary to take into account the dissi- 

 pation which happens during the interval of observa- 

 tion. The preceding method was employed by Cou- 

 lomb, when he wished to determine the relation between 

 the quantities of electricity in two bodies ; but, in order 

 to measure the electric density in each point of a con- 

 ductor, he had recourse to the following ingenious me- 

 thod. 



In the small balance with which he made his first ex- 

 periments, he suspended his needle by a fine silver wire. 

 He then formed a cylinder of gum lac, bent as at cde 



Coulomb's 



tricity. 



Electricity. 



I'l. vi'r 

 ('('XI. V. 

 Fig. 3. 



Couluiub's 

 method o 

 measuring 

 the electric 

 density of 

 any p;irt of 

 a botly. 



Method of 

 making al- 

 lowance for 

 the dissipa- 

 tion by the 

 contact of 

 air. 



in i'ig. 3. and of the thickness of a hair, and to the end 

 of it he attached a circle of gilt paper c. After having 

 electrified the disc carried by the needle with an insula- 

 ted pin as before, he electrified the body upon which 

 the experiments were to be made, and then touched the 

 circle e with the point of the body, whose electrical 

 density he wished to ascertain. He then placed this 

 circle in the balance, and ascertained the quantity of its 

 electricity. Now as this circle is only five or six lines 

 in diameter, and the ISth part of a line thick, the elec- 

 trical density which it acquires by the tangential con- 

 tact, is cither the same with that of the point which it 

 touches, or proportional to that density. Hence by ma- 

 king this circle touch different points of a conductor in 

 succession, and by presenting it after every contact to the 

 needle of the balance, taking care that it shall always 

 have the same position, Coulomb was enabled to ascer- 

 tain the electrical densities of different points of conduc- 

 tors. 



In the comparison of succeeding observations, we 

 must obviously take into account the dissipation of elec- 

 tricity by the contact of air, and the method by which 

 this has been done by Coulomb is remarkably simple 

 and ingenious. In order to compare tlie electrical den- 

 sity of two points, he first touched one of the points by 

 the paper circle, and determined its density by placing 

 the circle in the balance as before. He then touched 

 the second point, and determined its density in a simi- 

 lar manner ; and after an interval equal to that which 

 had elapsed between the two touehiugs, he touched tin- 

 first point anew, and again determined its density, which 

 was of course less than before, from the effect of dissipa- 

 tion. He then took the mean between the two densi- 

 ties found for the first point, which was therefore the 

 real density at the instant of the second observation, so 

 that die two densities thus measured were free from any 

 error arising from dissipation. 



This method, though in general the most conveni- 

 ent and simple, as well as the most exact, is never- 

 theless affected with an error in practice, arising from 

 the gum lac not being perfectly impenetrable to the 

 electric matter. This error is greatest in damp we.i- 

 ther, and with impure gum lac, die least clear being 

 generally the most impenetrable by the electric mat- 

 ter. In order to get rid of this error, it is neces- 

 sary always to try its non-conducting power, by touch- 

 ing the electrified body with the extremity e, and ob- 

 serving if it has any effect upon the needle of the ba- 

 lance; if any scnsiblechange is produced upon the needle, 

 the cylinder of gum lac must be rejected. With these 

 precautions, Coulomb made the following experiments. 



4. On the Distribution of Electricity between two Globes 

 of different Diameters, and in Contact. 



Having placed in the great balance, Fig. 1, an elec- On the iTs- 

 trified globe six inches and three lines in circumference, tribution ui" 

 he observed the force of torsion which was necessary to ilectricity 

 bring back the needle to a distance of thirty degrees b j-' tw n '' 

 from the globe. He then immediately touched the first 1^,, 

 globe with another globe 24. inches in circumference, diameters, 

 and observed a new force of torsion necessary to bring and in con- 

 it back to the same distance : The following were the re- tact - 

 suits which he obtained. 



