1891.] vibrating electrical system, and its radiation. 169 



When the system is disturbed the liquid will sway backwards 

 and forwards between the condenser coatings in a way which 

 gives a very real representation of the actual electric oscillation. 



The only element in this representation that is not easily 

 realisable is the skin, of rigidity great compared with that of 

 the solid ; if however the solid were a jelly the skin would be 

 naturally provided by supposing the system represented in the 

 diagram to be constructed of very flexible sheet metal. 



The greater the capacity of the condensers compared with the 

 section of the connecting wire the longer is the period of the 

 graver vibrations ; thus illustrating the dependence of the period 

 on the capacity and self-induction of the vibrator. There are 

 also overtones in which the pulse of normal displacement is 

 almost confined to the connecting wire, the greater mobility at 

 its ends making those points approximately nodes ; and their 

 nodal character will be the more prominent the greater the 

 capacity per unit area at the points where they are attached to 

 the condensers. For these overtones the half wave-length is thus 

 a sub-multiple of the length of the connecting wire. 



Owing to the small surface of the connecting wire these over- 

 tones would not have much chance of being communicated to the 

 surrounding medium, except by reason of the general principle 

 which requires that the shorter the period for given dimensions of 

 the vibrator the more of the vibrational energy travels outwards 

 into the medium, and that in a ratio which increases very rapidly 

 with increasing frequency. In the Hertzian oscillator the con- 

 densers are replaced by large metallic plates, and it seems clear 

 that the waves that are the chief subject of experiment issue from 

 the large surfaces afforded by these plates, and so belong to the 

 lower periods of the vibrator. 



The waves of a Hertzian vibrator are therefore radiated from 

 the plates of the vibrator : to obtain considerable radiation to a dis- 

 tance it is essential that the dimensions of a plate should be con- 

 siderable compared with the length of a wave of the radiation. 

 This condition would not be fulfilled if the plates were replaced by 

 condensers, for though the energy of the vibration would thereby 

 be increased, its wave-length for the fundamental periods would be 

 lengthened, and the radiation, therefore, very much diminished. 

 Condensers would be allowable instead of plates when the disturb- 

 ance is guided along conducting wires, but as a rule they would 

 give only slight undulations in the dielectric at a distance from 

 conductors. 



The plates are thus like two poles radiating in opposite phases : 

 and the general equations given by Kirchhoff to represent this 

 type of motion are those used by Hertz in his discussion of the 

 general character of the radiation at a distance from the vibrator. 



