126 Prof. J. Zeleny on the Ratio of the Velocities of the 



A smaller number of the ions reach the gauze because more 

 are lost on the way by recombination, similarly as when the 

 ions move slower due to a smaller E.M.F. used. 



In addition, in this case, any accumulation of ions is opposed 

 by their motion sidewise out of the current, due to their mutual 

 repulsion to those parts where the accumulation is less. 



Again, if the velocity of the ions is but a little more than 

 that of the opposed blast, some of the ions which tend to 

 move against the gas-stream are blown away by the blast, 

 since their presence, if moving in the other direction, would 

 decrease the electric force below that sufficient to force the 

 ions against the stream. 



When the ions are moving to the gauze T against the gas- 

 stream, none of them can pass between the meshes of the 

 gauze into the tube E, as there is no electric force in that 

 direction ; but if the ions are moving with the stream, then 

 some and even most of them can be blown through the gauze, 

 to the wires of which the electric forces tend to draw them. 



§ 3. A Form of Tuhe for Constant Radiation. 



It is of the utmost importance, if accurate work is to be 

 done, to have a constant source of the Rontgen rays. 



Among a large number of tubes tried for this purpose one 

 type has proved far better than the rest. 



The tubes of this class which I have so far employed have 

 maintained a satisfactory constancy, until by prolonged use 

 the glass became quite dark from the metal sputtered from 

 the electrodes. 



The general form of the tubes used is shown in fig. 3. 



Fig. 3. 



The concave aluminium cathode is situated in the tube B C 

 while the anode, made of a sheet of platinum backed by an 

 aluminium disk, is situated in the bulb F. The parts back of 

 both electrodes are covered by tightly fitting tinfoil. The 

 little bulb D contains a small piece of charcoal which helps to 

 maintain a constant vacuum by the state of equilibrium 

 between the free and the absorbed gas. With the use of a 



