Aftion of Ekftrophorus explained. [Book IV. 



If an Apparatus of this kind is joined to one of thofe 

 conducting rods, erected to protect buildings from 

 the effects of lightning, it will ferve to give notice of 

 the approach and paflfage of an electrical cloud. 



It is remarkable, that in certain cafes bodies elec- 

 trified will retain their electric power for almoft any 

 length of time, and on this principle a very ingenious 

 inftrument has been conftructed, called an electro- 

 phorus. This machine confifts merely of a mafs of 

 refinous matter, contained in a box for the conveni- 

 ence of carriage, and a plate of metal fitted to com- 

 municate with it, which is lifted by a handle of glafs^ 

 or fome non-conducting fubftance. The refinous 

 mafs being rubbed with a flannel, or even with the 

 hand, and the plate of metal being applied to it, the 

 metal will become charged, and give out fparks very 

 freely to any conducting body ; and this property of 

 communicating electricity the refinous mafs will retain 

 for a length of time, without any frefh application 

 whatever. 



To explain thefe phenomena it will be again necef- 

 fary to recur to what has been faid concerning nega- 

 tive and pofitive electricity ; it will be necefiary alfo 

 to recollect, that the negative electricity was originally 

 termed the reftnous, beeaufe it was firft thought to be 

 peculiar to thofe fubftances. In the electrophorus, 

 therefore, the lower plate, or refinous mafs,. being ne- 

 gatively electrified, the matter is taken from the metal 

 plate, and this becoming alfo negatively electrified, 

 the fluid is attracted from any body which is pre- 

 fented to it. 



Several inftruments have been invented for meafur- 

 ing the quantity of electricity contained in any body. 

 Thefe generally are formed upon the principle of the 

 electric attraction, and conflft of a fmall pith ball, or 



other 



