364 GREAT MEN OF SCIENCE 



Sparks and of the electric arc; finally we have the still shorter 

 waves of the X-rays and gamma rays, the latter being given 

 off by radio-active bodies, and their length being measured 

 in millionths of a millimetre. 



Since the correctness of Maxwell's equations was now 

 placed beyond doubt, it was also clear that they should state 

 correctly the limits of validity of single laws discovered 

 earlier, such for example as Coulomb's law for the force be- 

 tween electrified bodies. This law only relates to stationary 

 electricity, as can be easily recognised. If, for example, a 

 quantity of electricity is situated not far from an oscillator, 

 it is exposed to forces which can be more or less correctly cal- 

 culated by Coulomb's law, in spite of the motion of the elec- 

 tricity upon the oscillator; but if the quantity of electricity is 

 half a wave-length away from the oscillator, the forces have 

 changed their direction; they would now be calculated by 

 Coulomb's law wrongly, not only in respect of their magni- 

 tude, but also their direction. This also renders clear the 

 fact that 'actions at a distance,' determined in a fixed manner 

 by the distance, are non-existent in these cases. On the 

 contrary, the forces exerted by the electricity, and proceed- 

 ing from it, spread out into space, and require time for this 

 purpose; and although the propagation takes place with the 

 velocity of light when the motion of the electricity is very 

 rapid, and particularly when it is of a to-and-fro character, as 

 in the case of the oscillations, the delay becomes easily per- 

 ceptible. This propagation with delay is alone that which 

 causes waves to be formed when electricity oscillates. At 

 the same time the forces, though Coulomb's law fails to 

 represent them, are quite correctly given by Faraday's lines 

 of force, only these lines of force must have ascribed to them 

 the property of moving in a direction at right angles to their 

 own direction, with the velocity of light. The exact manner 

 in which this happens, and the forms thereby assumed by the 

 lines of force, are correctly given by Maxwell's equations, a 



