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A CENTURY OF SCIENCE 



must underlie electrodynamics. Lorentz had shown that 

 a rigorous solution of the electrodynamic equations did 

 away entirely with Maxwell's displacement current, but 

 made the electromagnetic field at a point in space depend 

 not upon the distribution of charges and currents at the 

 same instant, but at a time earlier sufficient to allow the 

 effect to travel with the velocity of light from the charges 

 and currents producing the field to the point at which the 

 electric and magnetic intensities are to be found. The 

 position of a charge or current element at this earlier time 

 he denoted its "effective position." The effective distri- 

 bution, then, is that actually seen by an observer stationed 



FIG. 1. 



FIG. 2. 



FIG. 3. 



at the point under consideration at the instant for which 

 the intensity of the electromagnetic field is to be deter- 

 mined. This solution of the electrodynamic equations 

 led in turn to rigorous expressions for the electric and 

 magnetic intensities produced by a very small charged 

 particle, such as an electron. Fig. 1 shows the electro- 

 static field produced by a charged particle at rest. The 

 lines of force spread out radially and uniformly in all 

 directions. In fig. 2 the electron is supposed to have a 

 velocity v horizontally to the right of an amount smaller 

 than, though comparable with, the velocity of light c. 

 It is seen that the lines of electric force still diverge 

 radially from the charge, but are crowded in the equato- 

 rial plane and spread apart in the polar regions. The 

 dissymmetry grows as the velocity increases until if the 

 velocity of light should be reached the field would be 

 entirely concentrated in a plane at right angles to the 

 direction of motion. Now it may be shown that fig. 2 is 



