CONTACT-ELECTRICITY. 647 



ditions; first, it ought to be so large as not to be melted 

 or ruptured if the discharge passes ; second, it must be 

 considerably higher than the highest or most projecting 

 parts of the building which it is to protect; third, its 

 lower extremity must be in well-conducting connection 

 with the ground. It should, therefore, not be led into dry 

 soil, but if possible into a well or brook in the neighbour- 

 hood of the building. When such a termination cannot be 

 had, the foot of the conductor is usually sunk into a deep 

 hole filled with wood-ashes, which conduct very well.. 



B. Contact- Electricity. 



48. Contact- Electricity. Galvanic Element. Galvanic 

 Current. Friction and induction are not the only 

 modes of developing electricity; it may also be produced 

 by the contact of certain bodies of different kind. This 

 mode of generating electricity was discovered by the 

 observations of Galvani, and electricity produced by 

 contact of bodies is hence often called 4 Galvanic 

 Electricity.' The electricity developed by contact is 

 essentially of the same nature as the electricity pro- 

 duced by friction, but there exist important differences 

 between them, both as regards the quantity that can be 

 accumulated on a given conductor, and also as regards 

 the quantity that can be produced in a given time. 

 As a consequence of these differences, many effects may 

 be more easily produced by frictional than by galvanic 

 electricity, and, rice versa, effects of galvanic electricity 

 can only with great difficulty be obtained by frictional 

 electricity. 



When two different metals are partially immersed 



