CHAP, i.] ELECTRICITY AND MAGNETISM. 9 



Electricity may either reside upon the surface of bodies 

 as a charge, or flow through their substance as a 

 current. That branch of the science which treats of 

 the laws of the charges upon the surface of bodies is 

 termed electrostatics, and is dealt with in Chapter 

 IV. The branch of the subject which treats of the flow 

 of electricity in currents is dealt with in Chapter III., and 

 other later portions of this book. 



8. Conductors and Insulators. The term "con- 

 ductors," used above, is applied to those bodies which 

 readily allow electricity to flow through them. Roughly 

 speaking bodies may be divided into two classes those 

 which conduct, and those which do not ; though very 

 many substances are partial conductors, and cannot well 

 be classed in either category. All the metals conduct 

 well ; the human body conducts, and so does water. 

 On the other hand glass, sealing-wax, silk, shellac, gutta- 

 percha, indiarubber, resin, fatty substances generally, 

 and the air, are " non-conductors." On this account 

 these substances are used to make supports and handles 

 for electrical apparatus where it is important that the 

 electricity should not leak away ; hence they are some- 

 times called insulators or isolators. Faraday termed 

 them dielectrics. We have remarked above that Gil- 

 bert gave the name of non-electrics to those substances 

 which, like the metals, yield no sign of electrification when 

 held in the hand and rubbed. We now know the reason 

 why they show no electrification ; for, being good conduct- 

 ors, the electricity flows away as fast as it is generated. 

 The observation of Gilbert that electrical experiments 

 fail in damp weather is also explained by the knowledge 

 that water is a conductor, the film of moisture on the 

 surface of damp bodies causing the electricity produced 

 by friction to leak away as fast as it is generated. 



9. Other electrical effects. The production of 

 electricity by friction is attested by other effects than 

 those of attraction and repulsion, which hitherto we have 



