8 PROPERTIES OF AN ELECTRIC CHARGE [CH.L- 



curvature of path ? Something more* than simple 

 electrostatics and simple magnetism is then observed. 

 For whenever a conductor is moved across a 

 magnetic field it is well known that an electromotive 

 force acts in that conductor, of magnitude equal to 

 the rate at which magnetic lines of force are being 



cut ; or in symbols . 



E= dN/dt, 



which is the fundamental ' dynamo ' equation. This 

 is called the phenomenon of magneto-electric in- 

 duction ; it is the induced E.M.F. discovered by 

 Faraday, and it necessarily occurs whenever magne- 

 tism and relative motion are superposed. 



It is quite independent of the conductivity of 

 the conductor however, and would have the same 

 value if the motion took place in an insulator, 

 though of course it could not then produce the 

 same effect as regards conduction-currents. 



The effect of a conductor is to integrate, or add 

 up, the E.M.F.'S generated in each element all along 

 its length, and thus to display the effect in an 

 obvious manner : especially when the conductor is 

 made very long and is compactly coiled (as in an 

 armature). The definition of electromotive force 

 between two points A and B, or round any closed 

 contour, is the line-integral of electric field from A 

 to B, or round the same contour. In the unclosed- 

 path case it is measured by the difference of electric 

 potential between A and B. 



One of the easiest and most ordinary ways of 

 superposing motion and magnetism, is to allow or 

 cause a magnetic field to vary in strength (as in 

 a Ruhmkorff coil) ; for then the lines of force move 

 broadside on, expanding or contracting as the case 

 may be, and thus at once we get the phenomenon 



