A CENTURY'S PROGRESS IN PHYSICS 349 



wave length was measured. Hertz calculated the fre- 

 quency of his radiator from its dimensions, and then 

 computed the velocity of the disturbance. In spite of an 

 error in his calculations, later pointed out by Poincare, 

 he obtained very nearly the velocity of light for waves 

 traveling through air, but a velocity considerably smaller 

 for those propagated along wires. Subsequent work by 

 Lecher, Sarasin and de la Rive, and Trowbridge and 

 Duane (49, 297, 1895; 50, 104, 1895) cleared up this dis- 

 crepancy, and showed the velocity to be in both cases 

 identical with that of light. The last-named investiga- 

 tors increased the size of the oscillator until it was possi- 

 ble to measure the frequency by photographing the spark 

 in the secondary with a rotating mirror. The positions 

 of nodes and loops were obtained by means of a bolom- 

 eter after the secondary had been tuned to resonance 

 with the vibrator. The velocity thus found for electro- 

 magnetic waves along wires is within one-tenth of one 

 percent of the accepted value of the velocity of light. 

 Hertz's later experiments showed that waves in air suf- 

 fer refraction and diffraction, and he succeeded in 

 polarizing the radiation by passing it through a grating 

 constructed of parallel metallic wires. 



In order to satisfy the law of action and reaction, it 

 is found necessary to attribute a quasi-momentum to 

 electromagnetic waves. When a train of such waves is 

 absorbed, their momentum is transferred to the absorb- 

 ing body, while if they are reflected an impulse twice as 

 great is imparted. This consequence of theory, foreseen 

 by Maxwell and developed in detail by Poynting, Abra- 

 ham and Larmor, has been verified by the experiments of 

 Lebedew, and Nichols and Hull. 4 The latter used a deli- 

 cate torsion balance from which was suspended a couple 

 of silvered glass vanes. In order to eliminate the effect 

 of impulses imparted by the molecules of the residual 

 gas, such as Crookes had observed in his radiometer, 

 readings were made at many different pressures and the 

 ballistic rather than the static deflection recorded. 

 After the pressure produced by light from a carbon arc 

 had been measured, the intensity of the radiation was 

 determined with a bolometer. Preliminary experiments 

 indicated the existence of a pressure of the order 



