Maiiiptilution of the Ekclrical Double f. 297 



eeptible when the plate is lifted up. Mr. Cavallo improved this indrument, by communi- 

 cating the eleflricity of the raetallic plate, when raifed up, to another plate of the fame kind, 

 but much fmaller, and in contail with an imperfeclly conducing furface. And the Rev. 

 Air. Bennett adapted the fame to.liis eleftrometer *. This is the condenfer of Volta; and 

 the imperfeclly coiiduding plate may be a piece of dry marble, or a common wooden table, 

 or any kind of plate covered with thin filk. 



Mr. Bennett was, I believe, the firft who improved the condenfers of Volta, by making 

 ufe of a manipulation fimilar to that of the Profeflbrs Lichtemberg and Klincock. His 

 inflrument, called the doubler, confifts of three metallic plates, capable of being applied to 

 each other with their flat furface, but prevented from contad in this fituation by a thia 

 coating of varnidi. They have infulating handles at the fide, and may be brought into adual 

 contaa edgewife. Let the three plates be called A, B and ,C ; and the manipulation for in- 

 creaGng the intenfity of one of them, for example A, will be as follows : 



The plate A being infulated, place B upon it. In this fituation communicate eledlricity 

 to it, while B is touched with the finger. The confequence will be, that A will receive a 

 much larger quantity of eleftritity than it would elfe have abforbed if B had' not been 

 prefent. 



Remove the communication from A, and the finger from B, and raife the latter by its 

 infulating handle. A and B will then e.Uiibit the oppofite eledlric flates more ftrongly ' 

 than while together. 



Place C upon B, and touch C with the finger. It will alTume the oppofite ftate to B, or, 

 in other words, the fame ftate as A. 



Place B upon A, as before, touching B with the finger ; and at the fame time apply C 

 edgewife to A. In this fituation, C, being without conipenfation, will give the greateft 

 part of its eleftricity to A. 



Remove C; take the finger from B, and feparate B from A. The oppofite eleflricitles 

 will be ftronger than before, on account of the increafe afforded by C. 



Place C upon B again, as in the third ftage of the original procefs, and repeat the fub- 

 fequent manipulations'. In each of them the intenfities of A and B will be nearly doubled. 



Though this procefs is fimple and evident, yet it requires to be learned, and takes a cer- 

 tain time for the performance. It therefore appeared a defirable objedl to form an inftru- 

 ment which lliould by fome very fimple movement complete this fmall feries of operations. 

 In the month of December 1787 Mr. Partington lent me an inftrument contrived by Dr. 

 Darwin, and confifting of four metallic plates, two of which were moveable by wheel-work 

 into pofitions which required them to be touched with the hand in order to produce the 

 effe£t. It appeared to me that the whole operation, including the touching, might be done 

 by a fimple combination without wheel-work by the dire£l: rotation of a winch. This was 

 foon afterwards efleiSled, and communicated by me to the Royal Society in the year 1783. 

 The inllrument is alfo defcribcd in INIr. Bennett's New Experiments en Eleclridiy-, publilhed 

 in 1789, and in other books. 



Mr. Bennett and Mr. Cavallo obferved, foon after the difcovery of the doubler that it 

 never fails to cxiiibit an eledtric Itate by the mere operation, without any communication 



♦ Philofophical Tranfaaions, LXXVIi. p. 31. 



of 



