PREFACE 



IT was intended that the subject of Magnetism and Electricity 

 should be a volume complete in itself, of the Series forming a Text- 

 Book on Pnysics, but it has been found necessary to divide it into 

 two. The present volume contains an account of the chief pheno- 

 mena of electric and magnetic svstems when they are respectively 

 charged and magnetised. The effects of changes in the systems are 

 only considered >taticallv, after the changes are effected, and the 

 sv-teniN have become steady again. The phenomena accompanying 

 the progiv.>s of change' belong lo electric current or electro- 

 magneti>m ami will be treated in another volume. 



The view of electric action taken by Faraday and largely deve- 

 loped in mathematical form bv Maxwell still holds good. Faraday 

 showed that we must regard the material medium between 

 electrified bodie> a> in an altered condition. In this volume the 

 quantity which is taken a> measuring the alteration is termed 

 "electric strain." The term is adopted in preference to "electric 

 displacement" or "electric polarisation," in that it does not 

 <>r imply any special hypothesis. When the- term is first 

 found useful, it is not ncce>sarv to form a hypothesis as to the 

 nature of the alteration, though the somewhat vague representation 

 of it as the beginning of chemical separation, the drawing apart of 

 two equally and oppositely charged portions of atoms or molecules, 

 which Faradav appears to adopt, is helpful. The electron theory 

 i^ substantially, a much more highly developed and dynamical form 

 of Faraday's theory. But the electron theory was suggested by the 

 phenomena of electric discharge, and is so largely a current theory 

 that it is more appropriately considered in the companion volume. 

 In this volume it is only introduced occasionally in magnetism. 



