218 MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



fixed, and the distances between their points of discharge accurately measured by 

 means of a micrometer-screw and index at s, comparative quantities may be always 

 estimated and restored from time to time with a great degree of accuracy. 



14. Comparative quantities of electricity may be transferred to simple conductors, 

 by abstracting sparks from an insulated jar, D, fig. 6. charged with a given accumu- 

 lation by the preceding process. The sparks may be taken immediately on the con- 

 ductor, or otherwise, on an insulated transfer plate, p, fig. 7, of given capacity, and 

 then deposited on the conductor, as in a, fig. 2. This method of estimating quantity 

 is extremely eflS^cient in researches with simple conductors. The following experi- 

 ments in illustration of it are not unimportant : 



(a.) An insulated metallic disc, a, fig. 2, electrified many times in succession by a 

 series of sparks transferred to it from the charged jar D by means of the insulated 

 plate p, was found at each transfer to be electrified to so nearly the same amount, 

 that the differences were not apparent on the electrometer, fig. 2, or on the electro- 

 scope, fig. 1 ; the disc being supposed in contact with either of these instruments. It 

 is only requisite in this case to restore the opposite coating of the jar to its previous 

 state, after each contact with the transfer plate *. 



When a portion of the charge is abstracted so as to sensibly decrease the quantity 

 in the jar, a new point may be arrived at, from whence another series of sparks can 

 be obtained of less m^-gnitude, but differing extremely little in quantity as compared 

 with each other ; and this process may be continued to a low point of accumulation 

 in the jar. 



(d.) The quantity given off by the positive coating will depend on the dimensions 

 of the conductor to be charged, and on the state of the negative coating : thus a con- 

 ductor of a double capacity becomes charged by a single contact with a double quan- 

 tity ; a conductor of a treble capacity, with a treble quantity (56.) ; and generally, 

 conductors varying in superficial dimensions are electrified by one contact, in such 

 way as to exhibit precisely the same force when connected with the electrometer. 

 The extent of this action is considerable, provided the opposite coating be placed in a 

 sufficiently free state. 



15. It would seem by these experiments, that in the discharging of a charged jar, 

 by the successive abstraction of small sparks, series may be obtained of such slow 

 convergence, that certain terms near each other may be taken as equal ^. 



* It is of no consequence to the experiment what part of the electrical conductor touches the contact ball of 

 the electrometer ; the same force is invariably indicated whether we make the contact at either of its extremi- 

 ties or centre. 



t In the various experiments with simple conductors, described in this paper, it is essential to remark, that 

 the most perfect system of insulation was requisite : all the glass rods were therefore as slender as possible, 

 and were varnished with a solution of shell-lac in alcohol. The experiments also have been carried on always 

 in a dry atmosphere, and the various insulations occasionally warmed with a stick of burning charcoal. The 

 success of this process is not a little remarkable : the index of the electrometer remains, as it were, fixed on a 

 given point for a comparatively long period of time ; hence the results are decisive. It is, on the contrary, 



