VOL. LXXVIII.] PHILOSOPHICAL TRANSACTIONS. 350 



viate the weighty objections that have been mentioned in the preceding pages, the 

 first desideratum being to construct a set of such plates as, when no electricity is 

 communicated, they will produce none after having performed the operation of 

 doubling for a certain number of times. On the whole, the methods by which 

 small quantities of electricity may be ascertained with precision are, he says, only 

 3. If the absolute quantity of electricity be small and pretty well condensed, as 

 that produced by a small tourmalin when heated, or by a hair when rubbed, the 

 only effectual method of manifesting its presence, and ascertaining its quality, is 

 to communicate it immediately to a very delicate electrometer, viz. a very light 

 one, that has no great extent of metallic or of other conducting substance ; be- 

 cause if the small quantity of electricity that is communicated to it be expanded 

 throughout a proportionably great surface, its elasticity, and of course its power 

 of separating the corks of an electrometer, will be diminished in the same pro- 

 portion. The other case is, when we want to ascertain the presence of a con- 

 siderable quantity of electricity, which is dispersed or expanded into a great space, 

 and is little condensed, like the constant electricity of the atmosphere in clear 

 weather, or like the electricity which remains in a large Leyden phial after the 

 first or 2d discharge. 



To effect this, he uses an apparatus, which in principle is nothing more than 

 Mr. Volta's condenser ; but with certain alterations, which render it less effica- 

 cious than in the original plan, but at the same time render it much less subject 

 to equivocal results. He places 2 of the above described tin plates on a table, 

 facing each other, and about -f of an inch asunder. One of these plates, for in- 

 stance A, is connected with the floor by means of a wire, and the other plate b is 

 made to communicate, by any convenient means, with the electricity required to 

 be^llected. In this disposition the plate b, on account of the proximity of the 

 other plate, will imbibe more electricity than if it stood far from it, the plate a 

 in this case acting like the semi-conducting plane of M. Volta's condenser, though 

 not with quite an equal effect, because the other plate b does not touch it ; but 

 yet, for the very same reason, this method is incomparably less subject to any 

 equivocal result. When the plates have remained in that situation for the time 

 that may be judged necessary, the communication between the plate b, and the 

 conducting substance which conveyed the electricity to it, must be discontinued 

 by means of a glass stick, or other insulating body ; then the plate a is removed, 

 and the plate b is presented to an electrometer, to ascertain the quality of the 

 electricity ; but if the electrometer be not affected by it, then the plate b is 

 brought with its edge into contact with another very small plate, which stands 

 on a semi-conducting plane, after the manner of M. Volta's condenser ; which 

 done, the small plate, being held by its insulating handle, is removed from the 

 inferior plane, and is presented to the electrometer : and it frequently happens. 



