312 SCIENCE PROGRESS 



electrical wire system, and these waves also charged to a varying 

 potential an insulated plate on which the rays fell. The consequent 

 emission of secondary corpuscles from this plate was measured 

 by a Faraday cylinder connected to an electrometer ; the amount 

 was obviously controlled by the phase-relation between the 

 potential of the plate and that of the cathode of the Rontgen ray 

 bulb. If the various distances and wire lengths were adjusted 

 so that the charge received by the Faraday cylinder was (say) 

 a maximum, then it was found that if the distance of the X-ray 

 bulb from the insulated plate was altered by a certain amount, 

 the wire along which the waves travelled to the plate had to 

 be altered by the same amount to restore the maximum. Thus, 

 according to Marx, the Rontgen rays travel with the same 

 velocity as electric waves along wires, and therefore with the 

 velocity of light. 



Franck and Pohl (1908) have repeated Marx's experiments, 

 and question the validity of his method. Their criticisms, which 

 are supported by a large number of experiments, throw very 

 serious doubts on Marx's results. Their objections are : 



(i) That the potential change of the insulated plate was 

 insufficient to alter the emission of corpuscles by more than 

 ten per cent. 



(ii) That ions were produced directly by the Rontgen rays 

 in the gas round the insulated plate ; these were comparable in 

 amount with the electrons from the plate, and, being measured 

 with them in greater or less degree, confused the sharpness of 

 the readings. They found that the charge received by the 

 cylinder might indeed, by altering the gas pressure, be changed 

 in sign. 



(iii) That Marx's results were really due to the Himsredt 

 effect. Himstedt has shown that as one approaches a discharging 

 point subjected to an alternating potential one passes through 

 zones in which the charge in the air is first negative, then 

 neutral, and finally positive. These zones have positions de- 

 pending on the potential of the point and the gas pressure. 

 Marx's readings, so Franck and Pohl maintain, only indicated 

 that the amplitude of the waves was such as to bring the 

 Faraday cylinder into the neutral zone of the oscillations to 

 which the insulated plate was subjected. By altering the con- 

 ditions, Franck and Pohl were able to obtain widely varying 

 values for the velocity of the Rontgen rays. 



