INTRODUCTION. 



THE THREE DIVISIONS OF ELECTROMAGNETISM. 



1. THE fact that a piece of amber, on being rubbed, attracted to itself 

 other small bodies, was known to the Greeks, the discovery of this fact being 

 attributed to Thales of Miletus (640-548 B.C.). A second fact, namely, that 

 a certain mineral ore (lodestone) possessed the property of attracting iron, 

 is mentioned by Lucretius. These two facts have formed the basis from 

 which the modern science of Electromagnetism has grown. It has been 

 found that the two phenomena are not isolated, but are insignificant units in 

 a vast and intricate series of phenomena. To study, and as far as possible 

 interpret, these phenomena is the province of Electromagnetism. And the 

 mathematical development of the subject must aim at bringing as large 

 a number of the phenomena as possible within the power of exact mathe- 

 matical treatment. 



2. The first great branch of the science of Electromagaetism is known 

 as Electrostatics. The second branch is commonly spoken of as Magnetism, 

 but is more accurately described as Magnetostatics. We may say that 

 Electrostatics has been developed from the single property of amber already 

 mentioned, and that Magnetostatics has been developed from the single 

 property of the lodestone. These two branches of Electromagnetism deal 

 solely with states of rest, not with motion or changes of state, and are 

 therefore concerned only with phenomena which can be described as statical. 

 The developments of the two statical branches of Electromagnetism, namely 

 Electrostatics and Magnetostatics, are entirely independent of one another. 

 The science of Electrostatics could have been developed if the properties of 

 the lodestone had never been discovered, and similarly the science of 

 Magnetostatics could have been developed without any knowledge of the 

 properties of amber. 



The third branch of Electromagnetism, namely, Electrodynamics, deals 

 with the motion of electricity and magnetism, and it is in the development 

 of this branch that we first find that the two groups of phenomena of 

 electricity and magnetism are related to one another. The relation is 



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