298 Hon. R. J. Strutt on the Discharge of 



extends depends on the current through the tube. The greater 

 the current, the further it reaches towards the cathode. By 

 increasing the external resistance it can be made to recede 

 as far as the point A. But when this is done the current is 

 so small that it does not suffice to produce a continuous dis- 

 charge through the gas. The telephone emits a humming 

 noise, and of course no true measurement of the cathode-fall 

 is possible. When the external resistance is diminished so 

 that a continuous current is obtained, the potential-difference 

 between c and d increases with the current, although the 

 negative glow does not cover the whole cathode. This is due 

 to the added potential-difference of the variable length AB 

 of the positive column. 



Many different tabes of various diameters and with the gas 

 at very different pressures were tried, but the same trouble 

 occurred in all. In one tube the testing electrode A was 

 covered with a glass hemisphere open on the side facing the 

 cathode, thus (fig. 3j — 



Fig. 3. 

 testing wire 



Cathode 



It was hoped that this glass protection might keep off the 

 positive column, and enable good measurements to be made; 

 but the readings obtained were just as anomalous as before. 



The difficulty was finally overcome in the following manner: — 

 Instead of a wire cathode, a platinum disk was employed, of 

 such a diameter as to fill up as nearly as possible the whole 

 cross-section of the tube without, however, touching the 

 sides. It is clear that in this case the positive column cannot 

 get back behind the tesiing-point. The negative glow formed 

 a patch on the disk-electrode. The area of this patch depended 

 on the current. And as long as the glow did not cover the 

 whole disk, the cathode-fall was independent of the current. 

 The only objection to the disk is that it is difficult to pass a 

 sufficiently strong current through the tube to heat so large 

 an area satisfactorily. Thus it is difficult to get rid of the 

 occluded gases. 



This cause of error vitiated the earlier measurements. 



The cathode-fall in argon turned out to be so low that an 



