648 ELECTROMOTIVE FORCE. 



in a liquid which is a good conductor of electricity, 

 both metals become electrified, one of them positively, 

 the other negatively. The two electricities which 

 neutralised one another in each body are separated at 

 the moment of contact with the liquid, and to the force 

 which causes the separation, the name Electromotive 

 Force has been given. If the two metals are copper 

 and zinc, and the liquid water, the copper is charged 

 positively, the zinc negatively. The quantity of elec- 

 tricity in each metal is exceedingly small; it is many, 

 thousand times less, even when large plates of the two 

 metals are used, than the quantity of electricity in a 

 rubbed glass rod. The small charge being diffused 

 over a large surface, the electrical tension is accord- 

 ingly very small, and the electricity is incapable of 

 traversing even the smallest space filled with air, hence 

 no spark is produced. In order to prove the existence 

 of so feeble a charge directly by means of the electro- 

 scope, the instrument must be much more sensitive 

 than the common gold-leaf electroscope; in fact, special 

 instruments are required for the purpose, too expensive 

 and complex for description in this work. Neverthe- 

 less it is possible to collect, by means of a condensing 

 apparatus, so much of the electricity generated by the 

 contact of copper and zinc with a liquid, that its exist- 

 ence may be demonstrated with the gold-leaf electro- 

 scope. 



The apparatus used for the purpose usually called 

 simply a ' condenser/ while to other condensers special 

 names are given (Leyden jar, etc.) consists of two 

 circular plates of metal, having the surfaces which 

 are turned towards each other very evenly . ground 



