208 THE REALITIES OF MODERN SCIENCE 



In 1831 Michael Faraday performed his simple, 

 illuminating experiments on the production of currents 

 by electromagnetic induction. These phenomena he 

 pictured in terms of magnetic lines or tubes of force 

 which extend through space from the north pole of a 

 magnet to the south pole. Such lines, also, are im- 

 agined to form concentric circles in planes perpendicular 

 to a conductor carrying a current. At any point in 

 space their number is considered proportional to the 

 strength of the magnetic field and their direction that 

 of the field. If these lines are assumed to have two 

 mechanical properties they may be used to explain all 

 the attractions or repulsions of magnets and coils. 

 The first property is that of .contracting indefinitely. 

 The second is that of exerting a pressure on each other 

 which is perpendicular to their directions. 



Faraday's discoveries are usually expressed by saying 

 that a relative motion of a conductor and a magnetic 

 line induces an electromotive force. 1 An e.m.f. is in- 

 duced only while the conductor crosses lines of force, 

 and its amount depends upon the time rate at which 

 the lines are cut. If the conductor forms a closed cir- 

 cuit, then an induced current flows in it provided that 

 there is a net change in the number of magnetic lines 

 threading it. It makes no difference whether the 

 cutting is occasioned by the actual motion of the con- 

 ductor relative to a current-carrying coil, or magnet, 

 or is occasioned by changes in the current and hence 



1 The term electromotive force originated in the days when the 

 causes of motions were sought in forces, and persists to-day. It 

 is essentially synonymous with " difference of potential " in the units 

 of which it is measured. 



