324 PHILOSOPHICAL TRANSACTIONS. [aNNO 1772. 



conductor, or be let into the brass knob of ajar or battery, or set in a stand, lo 

 support itself. 



The moment that this little apparatus is electrified, the rod a is repelled by 

 the stem c, and consequently begins to move along the graduated edge of the 

 semicircle b ; so as to mark with the utmost exactness; the degree in which the 

 prime conductor, &c. is electrified, or the height to which the charge of any 

 jar or battery is advanced; and as the materials of which this little instrument is 

 made, are very imperfect conductors, it will continue in contact with any elec- 

 trified body, or charged jar, without dissipating any of the electricity. 



If it should be found, by trial in the dark, that any part of this instrument 

 contributes to the dissipation of the electric matter, which, when the electrifi- 

 cation was ver}' strong, I once observed mine to do, it should be baked* a little, 

 which will presently prevent it. If it be heated too much, it will not receive 

 electricity readily enough ; and then the motion of the index will not correspond 

 with sufficient exactness, to the <legree in which the body to which it is con- 

 nected is electrified; but this inconvenience is easily remedied, by moistening 

 the stem and the index, for the semicircle cannot be too dry. 



I find by experience, that this electrometer answers all the purposes I have 

 mentioned, with the greatest ease and exactness, I am now sure of the force 

 of any explosion before a discharge of a jar or battery, which I had no better 

 method of guessing at before, than by presenting to them a pair of Mr. Canton's 

 balls, and observing their divergency at a given distance: but the degree of diver- 

 gency was still to be guessed at by the eye, and the balls can only be applied oc- 

 casionally; whereas this instrument being constantly fixed to the prime con- 

 ductor or the battery, shows, without any trouble, the whole progress of the 

 charge; and, remaining in the same situation, the force of different explosions 

 may be ascertained with the utmost exactness before the discharge. 



If a jar be loaded with positive electricity, and I want to know the exact time 

 when, by attempting to charge it negatively, it first becomes discharged, I see 

 every step of its approach to this state by the falling of the index ; and the mo- 

 ment I want to seize, is the time when it has got into a perpendicular situation, 

 which may be observed, without the least danger of a mistake. Accordingly, 

 I find that, in this case, not the least spark is left in the jar. If I continue the 

 operation, the index, after having gained its perpendicular position, begins to 

 advance again, and thereby shows the exact quantity of the opposite electricity 

 that it has acquired. 



Considering the admirable simplicity, as well as the great usefulness of this 

 instrument, it is something surprising that the construction should not have oc- 



• Wanned a little, to dry off the damps, particularly from the index. — Orig. 



