336 L. Page — A Century's Progress in Physics. 



the time of Newton, planetary motions could be described 

 quite satisfactorily by means of the three laws of Kepler. 

 The motion of falling bodies on the earth's surface had 

 been described with a fair degree of accuracy by Galileo. 

 The value of Newton's law of gravitation, however, lay in 

 the fact that this great generalization made it possible to 

 describe these and many other types of motion by a 

 single simple formula, instead of leaving each to be gov- 

 erned by a number of separate and apparently unrelated 

 laws. The importance of such a generalization is meas- 

 ured by the economy of thought which it introduces. 



Electron Theory. — The electron theory was leading to 

 a reversal of Kelvin's idea that dynamical principles 

 must underlie electrodynamics. Lorentz had shown that 

 a rigorous solution of the electrodynamic equations did 



Fig. 1. 



Fig. 2. 



Fig. 3. 



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 

 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 



