356 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



Another alteration of the bottle electrometer was lately made by the Rev. Mr. 

 Bennet, and is described in the Philos. Trans, vol. T] . It consists principally in 

 substituting 2 slips of gold-leaf for the corks suspended by vi^ires. This alteration 

 has some peculiar advantages and disadvantages. Its advantages are in general a 

 greater degree of sensibility, and a more easy construction. Its disadvantages are, 

 first, that the instrument is not portable ; and, 2dly, that even when not carried 

 about, it is apt to be spoiled very easily. However, in some cases it is very use- 

 ful, so that on the whole it may be considered as a very good improvement. 



Besides the way of ascertaining small quantities of electricity by means of very 

 delicate electrometers, 2 methods have been communicated to the philosophical 

 world, by which such quantities of electricity may be rendered manifest, as could 

 not be perceived by other means. The first of these methods is an invention of 

 M. Volta, the apparatus for it being called the condenser of electricity, and is 

 described in the Phil. Trans, vol. 72. The second is a contrivance of the above- 

 mentioned Mr. Bennet, who calls the apparatus the doubler of electricity ; a de- 

 scription of which is inserted in the Phil. Trans, vol. T] . 



M. Volta's condenser consists of a flat and smooth metal plate, furnished with 

 an insulating handle, and a semi-conducting, or imperfectly insulating plane. 

 When we wish to examine a weak electricity with this apparatus, as that of the 

 air in calm and hot weather, which is not generally sensible to an electrometer, 

 we must place the plate on the semi-conducting plane, and a wire, or some 

 other conducting substance, must be connected with the metal plate, and 

 must be extended in the open air, so as to absorb its electricity ; then, after 

 a certain time, the metal plate must be separated from the semi-conducting 

 plane, and being presented to an electrometer will electrify it much more 

 than if it had not been placed on the above-mentioned plane. The principle 

 on which the action of this apparatus depends is, that the metal plate, while 

 standing contiguous to the semi-conducting plane, will both absorb and retain 

 a much greater quantity of electricity than it can either absorb or retain when 

 separate, its capacity being increased in the former, and diminished in the latter 

 case. Hence we find, that its office is not to manifest a small quantity of 

 electricity, but to condense an expanded quantity into a small space : so 

 that, if by means of this apparatus a person expected to render more manifest 

 than it generally is, when communicated immediately to an electrometer, the 

 electricity of a small tourmalin, or of a hair when rubbed, he would find himself 

 mistaken. 



It is Mr. Bennet's doubler that was intended to answer this end ; viz. to multi- 

 ply; by repeated doubling, a small, and otherwise unperceivable, quantity of elec- 

 tricity, till it became sufficient to affect an electrometer, to give sparks, &c. The 

 merit of this invention is certainly considerable ; but the use of it is far from pre- 



