for Electromagnetic Waves. 339 



an electroscope through a very small air-gap ; electric waves in the 

 vicinity will be detected by the electroscope if they are able to 

 excite oscillations of strength sufficient to break down the spark 

 gap. In the Fleming Oscillation Valve induced electric oscillations 

 are detected by their ability to impart unilateral conductivity to 

 the space between a cold cylinder and a hot carbon filament, the 

 conductivity being due to the negatively charged corpuscles 

 emitted from the heated carbon under the action of a directional 

 electric force in the oscillations. 



Comparative tests have been roughly made with the electric 

 detector and an iron filings coherer, and have proved quite 

 favourable to the former ; a short period electrometer (e.g. the 

 string electrometer described by Laby in Proc. Camh. Phil. Soc, 

 vol. XV.) seems especially suited for use in connection with the 

 present form of detector. 



It was observed also that in ionisation chambers such as those 

 described, distinct deflections would occur under certain conditions 

 of pressure and voltage in the absence of any electric waves pro- 

 duced by the induction coil ; as an example of one set of conditions 

 it was found that with the air at a pressure of '76 mm., and with 

 one electrode connected to the negative pole of a battery of 480 

 volts, frequent deflections occurred. The frequency of these deflec- 

 tions varied considerably with the inductance in the circuit ; in 

 fact, a small variation of the inductance changed the frequency 

 from about 100 to 12 deflections per minute. It is reasonable to 

 suppose that these deflections are due also to electric oscillations set 

 up in the circuit including the detector. The writer is making 

 further experiments with regard to these oscillations, especially 

 with the object of determining their origin. 



