L. Page — A Century's Progress in Physics. 315 



then turning to Ampere's work he shows how the 

 Lagrangian equations of motion lead to Faraday's law 

 if the single assumption is made that the magnetic 

 energy of the field is kinetic. In the treatment of open 

 circuits Maxwell's intuition led to a great advance, the 

 introduction of the displacement current. Consider a 

 charged condenser, the plates of which are suddenly con- 

 nected by a wire. A current will flow through the wire 

 from the positively charged plate to the negative, but in 

 the gap between the two plates the conduction current 

 is missing. So convinced was Maxwell that currents 

 must always flow in closed circuits, that he postulated an 

 electrical displacement in the medium between the plates 

 of a charged condenser, which disappears when the con- 

 denser is short-circuited. Thus even in the so-called 

 open circuit the current flows along a closed path. 



Maxwell's theory of the electromagnetic field is based 

 essentially on Faraday's representation by lines of force 

 of the strains and stresses of a universal medium. So it 

 is not surprising that he was led to a consideration of 

 the propagation of waves through this medium. The 

 introduction of the displacement current made the form 

 of the electrodynamic equations such as to yield a typical 

 wave equation for space free from electrical charges and 

 currents. Moreover, the disturbance was found to be 

 transverse, and its velocity turned out to be identical 

 with that of light. The conclusion was irresistible. 

 That light could consist of anything but electromagnetic 

 waves of extremely short length was inconceivable. In 

 fact so certain was Maxwell of this deduction from 

 theory that he felt it altogether unnecessary to resort to 

 the test of experiment. For the electromagnetic theory 

 explained so many of the details which had been revealed 

 by experiments in light, that no doubt of its validity 

 could be entertained. Even dispersion received ready 

 elucidation on the assumption that the dispersing 

 medium is made up of vibrators having a natural period 

 comparable 'with that of the light passing through it. 



Maxwell's book was published in 1873. Fifteen years 

 later, Hertz, 3 at the instigation of Helmholtz, succeeded 

 in detecting experimentally the electromagnetic waves 

 predicted by Maxwell's theory. His oscillator consisted 

 of two sheets of metal in the same plane, to each of which 



3 H. Hertz, Wied. Ann., 34, 551, 1888 et seq. 



