Short Electric Waves hy Air and Water- Vapour. 703 



greatest between *10 and '07 mm. The two coils were sepa- 

 rated by a distance o£ about 2*5 cm. The frequency of the 

 current in the outer coil was about 107, which corresponds 

 to a wave-length of about 30 metres. 



In Lecher's experiment, then, the ionization was very in- 

 tense, being of the nature of that accompanying a glow dis- 

 charge, and the absorption of the electromagnetic energy 

 took place in the immediate neighbourhood of the oscillating 

 system. Such an absorption could not well be spoken of as 

 an absorption of electric waves. In fact, the system used by 

 Lecher to produce the oscillations was a closed system, which 

 would not give off electric waves. 



In the case of Wireless Telegraph waves, the ionization of 

 the air which has to be considered is its natural ionization, 

 that is, the ionization produced by ligbt, the presence of 

 radium, and other causes always present. It is not produced 

 by the waves themselves since their electric field is too small 

 to produce ionization. 



In L908 the author and R. E. 0. Gowdy, in a paper read 

 before the American Physical Society *, described some ex- 

 periments made to detect the absorption of electric waves by 

 air at different pressures. Waves having a wave-length of 

 about 10 cm. were used, and sent through a tube 44*5 cm. 

 long, so that there were at least four complete waves between 

 the sender and the receiver. They stated that they thought 

 that they had observed two maxima of absorption of short 

 electric waves in air as the pressure was decreased from that 

 of the atmosphere to something less than one millimelre of 

 mercury. One maximum apparently occurred near the zero 

 of pressure, and the other between 40 and 60 cm. of 

 mercury. 



As there seemed to be some question as to the reliability of 

 these results, the work was continued by the senior author, 

 and the original results were not confirmed by later experi- 

 ments. In fact, it was found that the Duddell Thermo- 

 galvanometer used in the earlier experiments was not 

 sensitive enough to measure the disturbances due to the 

 5 cm. linear oscillator, and that the deflexions obtained must 

 have been due to some other cause. 



In the earlier experiments, the method adopted was to 

 send waves from the same oscillator through two separate 

 tubes, successively, one tube containing air at atmospheric 

 pressure, and the other, air at any desired pressure. The 

 waves were received by one and the same receiver. The 



* See abstract published in the Physical Review, xxvi. pp. 196, 197 

 (1908). 



