[ 3 6 ° ] 
giving it a quantity of the. contrary electricity. All 
thefe purpofes are anfwered, in the moft complete 
manner, by an electrometer of this gentleman’s con- 
trivance, a drawing of which I fend you along with 
the following defcription. 
The whole inftrument is made of ivory or wood, 
'[Tab. XI.] ( a ) is an exceeding light rod, with a cork 
ball at the extremity, made to turn upon the center 
of a femicircle (b)> and fo as always to keep pretty 
near the. limb of it, which is graduated: fc) is the 
uftem that fupports it, and may either be fixed to the 
prime conductor, or be let into the brafs knob of a 
.jar or battery, or fet in a (land, to fupport itfelf. 
The moment that this little apparatus is electrified, 
the rod (a) is repelled by the item fc), and confe- 
quently begins to move along the graduated edge 
of the femicircle (b) \ fo as to mark with the ut- 
-xnoft exadtneis, the degree in which the prime con- 
ductor, 5ec. is eledtrified, or the height to which the 
charge of any jar or battery is advanced ; and as the 1 
materials of which this little inftrument is made are 
■very imperfedt conductors, it will continue m contadl 
with any eledtrified body, or charged jar, without 
diffipating any of the eledtricity. 
If it fhould be found, by trial in the dark, that 
any part of this infcrument contributes to the diffipa- 
tion of the eledtric matter, (which, when the elec- 
trification was very firong, I once obferved mine to 
do) it fliould be baked * a little, which will preiently 
prevent it. If it is heated too much, it will not re- 
ceive eledtricity readily enough; and then the mo- 
tion of ihe index will not correfpond with fufiicien £ 
■* Wanned a little, to dry off the damps, particularly from 
•tlje judex. 
exattnefs. 
