A NEW ELECTROMETER DESCRIBED. 217 



above described, each degree corresponds to a variation of distance between the con- 

 ductors equal to the '01 of an inch. If, therefore, at the commencement of any given 

 experiment, we first bring the nearest points of the conductors m / in contact, the 

 index being in zero, and then depress the inferior conductor / a given distance, 

 known by means of the micrometer screw s, then all subsequent distances may be 

 readily determined between these points. 



11. It is now only requisite to observe, that the interior of the cylindrical counter- 

 poise p nis hollow, in order to weight it accurately, and cause it to hang vertically in 

 the water ; and there is a small hemispherical cup, p, fixed on its stem, for the recep- 

 tion of small adjusting weights *, by which the position of the index at of the scale 

 is regulated with great nicety. With respect to the form of the conductors ?nf, they 

 are generally plane circular areas, backed by small cones, and are of about two inches 

 diameter. Conductors of other forms, however, such as spheres and cylinders, may 

 be occasionally used when the object is to experiment more particularly on bodies of 

 peculiar forms. 



Experiments with this instrument are remarkably clear, notwithstanding the subtile 

 character of the principle we have to investigate : thus, when the insulations are per- 

 fect, and the atmosphere dry, the index immediately exhibits the amount of the 

 attractive force, and remains stationary for a much longer time than is required to 

 note the result. 



12. Considering that electrical inquiries would be much facilitated by an accurate 

 method of estimating comparative quantity, I endeavoured, as being essential to my 

 purpose, to obtain a unit of measure, and at length arrived at the following methods 

 of estimating quantity, which are simple and accurate. 



According to the known laws of electrical accumulation on coated jars, the quan- 

 tity accumulated on one coating is proportionate to the quantity given off by the 

 other : hence, if instead of transmitting the electricity evolved by the machine imme- 

 diately from its conductor, we communicate the charge from the outer coating of a 

 small jar furnished with a discharging electrometer, we may estimate pretty accu- 

 rately by the number of explosions, that is to say, by the number of charges which 

 have passed the smaller jar, the quantity accumulated. 



13. On this principle, I inverted a small jar, K, fig. 5, exposing about six inches 

 square of coating, on a brass rod fixed to the conductor of the machine, or otherwise 

 sustained on a separate insulation, and connected the jar or battery to be charged 

 with its outer surface, through the intervention of a brass ball, b. In this arrange- 

 ment, electricity is continually supplied to the jar, and the amount of the accumula- 

 tion accurately measured by the number of charges which the unit jar has received, 

 the charges being determinable by means of the discharging balls n n\ By diminish- 

 ing or increasing the distance between the discharging balls, the value of the unit 

 may be rendered as small or as great as we please : hence, if the balls be securely 



* Small lead shot may be employed for this purpose. 



