Vibrations in Field round a Theoretical Hertzian Oscillator. 247 



(2) After the writers had investigated the general theory of a double 

 point with damped intensity, an attempt was made to replace the well- 

 known Hertzian diagrams of the field by a more complete series, repre- 

 senting the field for seven complete oscillations, and showing how the 

 field for some twelve metres round the oscillator chosen, gradually 

 falls to nearly JQ of its maximum initial strength. These diagrams are 

 entirely due to Miss Alice Lee, and involved a large expenditure of 

 labour and time, which would, perhaps, not have been justified were 

 any other graphic representation of a damped wave motion available.* 

 These diagrams were originally intended for kinematograph representa- 

 tion, but that method of reproduction has not yet been found feasible. 



(3.) The writers next deal with the type of waves propagated, their 

 velocities and their phases. The following general conclusions are 

 reached : 



(i) Three waves of electro-magnetic force may be considered as sent 

 out from the oscillator, and not merely two as supposed by Hertz. 

 These are : 



(a) A wave of purely transverse electric force. 



(b) A wave of electric force parallel to the axis, briefly termed the 

 wave of axial electric force. 



(c) A wave of magnetic force. 



The waves of axial electric and of magnetic force move outwards 

 with the same velocity, which is, however, a function of the distance 

 from the centre of the oscillator. The intensity of both forces for 

 points on the same sphere varies as the cosine of the latitude, the polar 

 axis being the axis of the oscillator. 



The wave of transverse electric force is propagated with the same 

 velocity at all equal distances from the centre of the oscillator, but 

 this velocity differs from that of the two previous waves, the amplitude 

 is independent of the latitude, being constant over any sphere. The 

 velocity after the wave has reached a certain distance from the double 

 point is always greater than that of the waves of magnetic and axial 

 electric force. Its excess over the velocity of light tends to become 

 three times the excess of the velocity of the magnetic wave over the 

 velocity of light ; both the excesses decreasing asymptotically. 



(ii) The velocities of these waves undergo remarkable changes in 

 the neighbourhood of the oscillator, but these changes extend to dis- 

 tances which are greater than those within which a great proportion 

 -of Hertz's interference experiments were made. 



(iii) The point of zero phase for both transverse and axial electric 

 waves does not coincide with the centre of the oscillator, so that these 

 waves appear to start from spheres of small but finite radius round 



* It must be remembered that a damped wave motion does not denote merely a 

 factor e-Pi* in the wave intensity, but a factor e-JPiC*-*"'), which sensibly alters the 

 of the lines of force propagated. 



