PRESENT PROBLEMS 97 



and is therefore at zero potential. The ground contact of the tube 

 may be replaced by an air contact, and the negative terminal of the 

 machine may then be grounded on the gas-pipe if desired. In none 

 of these cases are the positive and negative currents delivered by the 

 machine superposed in the X-ray tube. In all these cases X-ray 

 effects are obtained, but in some respects the tube behaves very differ- 

 ently when in the positive current from what it does in the negative. 

 In the negative unipolar circuit, the cathode terminal of the tube 

 is in direct communication with the negative terminal of the machine. 

 When in the positive circuit, the anti-cathode terminal of the tube 

 is in direct communication with the positive terminal of the machine, 

 and the cathode terminal is acted upon inductively across the Crookes 

 tube vacuum. In the negative current the luminous appearances 

 are normal and stable. When in the positive current, the discharge 

 may be made to cease by holding the hands near the bulb, and the 

 luminous glow is affected by the motion of neighboring bodies. The 

 discharge is much more unstable. If the observer approaches the 

 suspended grounding device the face and the hands are covered by 

 luminous points of light, which characterize the cathode terminal. 

 This phenomenon is very striking under these conditions. Ball 

 discharges may be drawn from such point discharges on a metal point 

 to a photographic plate, moving on the plate towards the anode wire 

 or contact plate suspended in air. It is apparent that the wire, when 

 considerably removed from other bodies, is discharging upon the 

 dust particles in the air. 



In 1879 Spottiswoode and Moulton 1 published a paper containing 

 a great array of experiments upon the spark discharge through gases. 

 They there dealt with unipolar discharge, and their conclusions are 

 well worthy of notice in this connection. They conclude that " the 

 independence of the discharge from each terminal of the tube is so 

 complete that we can at will cause the discharges from the two 

 terminals to be equal in intensity but opposite in sign (as in the case 

 of the coil) or of any required degree of inequality (as in the case of 

 the coil with a small condenser). Or we can cause the discharge to 

 be from one terminal only, the other terminal acting merely recep- 

 tively (as in the case of the air-spark discharge with the Holtz ma- 

 chine) ; or we can cause the discharge to pass from one terminal only, 

 and return to it, the other terminal not taking any part in the dis- 

 charge; or finally, we can make the two terminals pour forth inde- 

 pendent discharges of the same sign, each of which passes back 

 through the terminal from whence it came." This work was done 

 before the Crookes tube had appeared. It is certainly interesting to 

 observe that when a high degree of rarefaction has been reached, 

 the activity within the tube is represented by the cathode stream, 



1 Phil. Trans., 1879. 



