B56 CONDUCTORS OF GALVANIC ELECTRICITY. 



electricity, only the best conductors can be easily 

 traversed by it. Through paper, wood, the skin of 

 the human body, and similar substances, current elec- 

 tricity is so little able to travel that these bodies be- 

 have in reference to it like insulators. Even pure 

 water is a bad conductor of galvanic electricity. 

 Metals, carbon, and solutions of salts and acids 

 (sulphuric, nitric, and hydrochloric), concentrated or 

 dilute, are the only bodies available for conducting 

 galvanic electricity. The solutions of salts and acids 

 are by far better conductors than pure water, but even 

 their conductivity is many hundred thousand, and even 

 many million times less than that of the metals. Of 

 all known substances silver is the best conductor of 

 galvanic electricity; copper stands next and very near 

 to it in conducting power. Copper wire is preferably 

 used for conducting galvanic electricity, not only on 

 account of its excellent conductivity, but also because 

 the malleability and ductility of the metal is so con- 

 siderable that it is easily brought into any desired form. 



When conducting wires of copper are connected with other bodies, 

 the metallic surface in contact must always be perfectly clean and 

 bright, free from verdigris or other deposits, because these conduct 

 electricity badly. The ends of copper wires which are used in 

 galvanic apparatus, or in experiments on galvanic electricity, should 

 therefore frequently be rubbed with sand-paper or scraped with a 

 blunt knife. 



Another condition needful for good conduction is that the con- 

 nected conductors should be kept in firm and close contact ; a mere 

 suspension or loose attachment of one body to another, as is permitted 

 in experiments on frictional electricity, would be altogether insuf- 

 .ficient and cause nothing but failure. The wires are therefore in 

 galvanic apparatus always fixed by binding screws. 



Various forms of such binding screws are represented in fig. 337. 

 The form A is most easily made. Into a piece of sheet brass, 45 mm 



