Applied Science 839 



stretched out into space. If these lines of force from one electri- 

 fied body fall on another electrified body, the tension in the strings 

 tends to draw the bodies together: we say the bodies attract one 

 another. But where the lines of force from one electrified body 

 end on another, we always find that these two bodies are charged 

 with different kinds of electricity. If one is charged with positive 

 electricity the other is charged with negative electricity. In this 

 case, as we have said, the two bodies attract one another. When 

 either a positive or a negative charge is produced, an opposite 

 charge is produced also. But suppose both bodies are charged 

 with the same kind of electricity, that is, they are both positive 

 or both negative. Then, as very simple experiments show, they 

 repel one another. The lines of force repel one another. 



We therefore see that, in studying any electrified body, we 

 have also to study the imaginary lines of force which radiate out 

 from it into the surrounding space. If we imagine a single 

 electrified body, say an electron, to exist in empty space, then 

 each line of force from the electron stretches out to an infinite 

 distance. The space surrounding the body is said to constitute 

 the electric field of the body. It is only quite close to the body, 

 however, that this field is of appreciable strength. Now suppose 

 that our electrified body, say a single electron, begins to move 

 and a current of electricity is simply electrons in motion what 

 happens? In the immediate neighbourhood of the electron its 

 lines of force are carried with it. But the lines of force do not 

 behave as if they were rigid spokes. The movement of the electron 

 is not communicated along the whole length of the lines of force 

 simultaneously. The lines of force require time to adjust them- 

 selves to the new conditions. We can picture to ourselves a kink, 

 due to the motion of the electron, travelling out along the lines 

 of force (see Fig. 1 facing p. 825). Now the startling discovery 

 was made that the velocity at which this kink travels is the 

 velocity of light. This travelling kink constitutes an electric wave. 

 The kink also produces a magnetic force. For this reason the 



