Mr Wellisch, An Elective Detector, etc. 337 



An Electric Detector for Electromagnetic Waves. By E. M. 

 Wellisch, B.A., Emmanuel College. (Communicated by Professor 

 Sir J. J. Thomson.) 



[Heceived 6 November 1909.] 



A series of experiments in connection with ionisation produced 

 b}^ collision was undertaken by the writer in order to test certain 

 theories which he had previously advanced with regard to the 

 passage of electricity through gases. The preliminary experiments 

 were made with an ionisation chamber containing two parallel 

 plane electrodes made of aluminium, and at a distance from one 

 another of 2 cm. ; one of these electrodes was connected to a 

 source of potential, the other to a Dolezalek electrometer. During 

 the course of these experiments it was observed that, when the 

 gaseous pressure and the electric field were so chosen that the gas 

 was on the verge of breaking down, a very feeble discharge of a 

 Rontgen ray bulb placed in the neighbourhood was sufficient to 

 produce an exceedingly large deflection of the electrometer needle 

 which had previously been stationary; the direction of the de- 

 flection indicated the passage of a transient electric current 

 through the gas. Further investigation of this electric charge 

 produced within the gas showed, however, that the determining 

 cause of the deflection lay not in the rays issuing from the Rdntgen 

 bulb, but in the electric oscillations set up by electric waves 

 proceeding from the induction coil which worked the bulb. The 

 Rontgen bulb was henceforth removed and a series of experiments 

 was conducted in order to ascertain what degree of sensitiveness 

 to electric waves could be obtained from similar arrangements. 

 Several forms of ionisation chambers were used, and each of them 

 proved sensitive as detectors of extraneous electric waves. The 

 accompanying diagram represents the scheme of connections 

 employed in one of the trials. In this case the detector consisted 

 of a glass tube T (about 4 cm. in diameter) ; the electrodes were 

 two plane parallel aluminium discs (each 2 cm. in diameter) at a 

 distance apart of 1 cm. The tube was connected to a Topler 

 mercury pump and a MacLeod gauge so that the pressure of the 

 gas (dry air) could be adjusted and measured. In the diagram B 

 represents the battery, one of whose poles is earthed, E the electro- 

 meter, R a high resistance (consisting of conducting glass) shunted 

 across the electrometer, G a variable capacity, and L a variable 

 self-induction. 



The sensitiveness of the detector depends naturally on a large 

 variety of circumstances ; in fact, in the preliminary trials, it was 



