of Lightning- Conductors. 229 



until the recoil spark B is as long as possible. Then measure, 

 and see whether the length of each wire is not equal to 



I hope to communicate some numerical results of observa- 

 tions made in this way to the British Association meeting at 

 Bath. 



It is interesting to see how short it is practically possible to 

 make waves of this kind. A coated pane can be con- 

 structed of say two centimetres electrostatic capacity, and, 

 bv letting it overflow its edge, a discharge circuit may be 



* -i n • -I • . 



provided of only a few centimetres electromagnetic induc- 

 tance. Under these circumstances the radiated waves will 

 be only some 20 or 30 centimetres long, corresponding to a 

 thousand million alternations per second. Some beautiful 

 diffraction experiments have been described by Lord Rayleigh 

 in a recent Friday evening discourse to the Boyal Institution 

 (reprinted in 'Nature, 5 June 1888), and some of these might 

 be used to concentrate the electromagnetic radiation upon 

 some sensitive detector — possibly one of Mr. Boys's radio- 

 micrometers, more likely some chemical detector — some pre- 

 cipitate or other that can be shaken out of solution by the 

 impact of long waves, or some of Captain Abney's photo- 

 graphic agents. 



Certainly the damping-coefficient B/2L is high, and the 

 radiation has a very infinitesimal duration ; but a rapid suc- 

 cession of discharges can be kept up by connexion with a 

 machine. 



Xo doubt much shorter waves still may be obtained by 

 discarding the use of any so-called condenser, and by causing 

 the charge in a sphere or cylinder to oscillate to and fro be- 

 tween its ends, as might be done by giving it a succession of 

 sparks. These oscillations, it is to be feared, however, would 

 have too small energy to be detected by ordinary means. If 

 thev could be made quick enough to affect the retina, no 

 doubt we could detect them with the greatest ease ; but it is 

 manifest that this can only be done by reducing the circuit to 

 a size less than the wave-length of light. The wave-length 

 of the electrical radiation is six times the mean of the induc- 

 tance and capacity, and each of these quantities is very com- 

 parable with the linear dimensions of the conductor concerned. 

 setting up electric oscillations in a body as small as a 

 molecule, no doubt they would be rapid enough to give 

 ordinary light-waves ; but the probability is that this is pre- 

 cisely what light-waves are. 



Either the atoms are made to vibrate relatively to the 



