1891.J the Discharge of Ley den Jars. 27 



This is characteristic of what I have elsewhere examined and 

 called " side flash." 



ESTIMATION OF WAVE-LENGTH. 



29. Although there is nothing precisely metrical about these experi- 

 ments, as so far conducted, it is well to notice that an approximation 

 to the self-induction of the main discharge circuit can be obtained 

 from them. For the capacity of the two gallon jars in cascade is about 

 28 K metres ; and if the length of the wires which give the best 

 recoil be taken as half a wave-length, the waves emitted are 60 metres 

 long. So the inductance of the discharge circuit can be got from 

 27T V /(28L/ y M,) = 60; whence L = 3*2^ metres. 



This is too small, showing that the waves are longer than 60 metres, 

 and that the most appropriate capacity for these particular wires 

 is something less than that of the two gallon jars in cascade.* 



HISTORICAL OBSERVATIONS. 



30. This evidence of the existence of electro-magnetic waves seemed 

 to me of considerable interest, because I had been for some years 

 contemplating the production of radiation by direct electro-magnetic 

 experiments, the difficulty being their detection, i.e., the proof of 

 their existence. 



My early notions, described to Section A of the British Associa- 

 tion at York (1881), were directed towards the ambitious attempt of 

 trying to make the waves short enough to be visible, at least to a 

 thermopile or to some chemical detector. But two years later, at 

 Southport, Fitzgerald pointed out that a discharging Leyden jar must 

 emit radiation, and that though its waves would be yards or miles 

 long, yet it might not be hopeless to prove that they were waves by 

 obtaining interference phenomena. 



Some of Lord Rayleigh's large-scale interference experiments with 

 sound waves, exhibited to the Royal Institution on January 20, 1888 

 ('Nature,' vol. 38, p. 208), re-awakened in me the hope that such 

 experiments were possible, and the desire to try them. And now, 

 simply by attaching long wires to a discharging Leyden jar circuit, 

 the waves had become without trouble conspicuous. One had only 

 to lengthen the wires enough, and to look at them in the dark, to see 

 by the brushes the nodes caused by the interference of the direct 

 and reflected pulses surging to and fro in the wires ; to see in fact 

 the waves themselves, and to measure their length in a manner pre- 

 cisely analogous to the well-known experiment of Melde. 



* [Or else that each wire behaves like an organ-pipe open at one end only, and so 

 is a quarter of a wave long. June, 1891. J 



