358 Dr.Balth. van der Pol on Measuring without Electrodes 



between the Lecher wires. The shaded curves at the bottom 

 of the figure indicate the approximate distribution o£ the 

 light in the tube, while the upper dotted curve N was ob- 

 tained in the same way (by sliding the tube again vertically 

 between the wires) but without a glow-discharge passing. 

 The latter observations were taken in order to examine the 

 influence of the disk electrodes on the tuning of the re- 

 sonator. It is seen that only when the electrodes came very 

 near the plane of the Lecher system did a small detuning take 

 place, observed as a slight decrease in the galvanometer 

 deflexion, so that this effect can in general be neglected. 



Turning to the curve M we see that the absorption of the 

 waves increases up to a point in the negative glow where 

 the absorption is maximum and therefore the current a 

 minimum. 



In the Faraday dark space the absorption gradually de- 

 creases in the direction of the positive column, while over 

 the striations the current is constant ; near the anode an 

 increase of current (decrease of absorption) occurs, and as 

 soon as the anode reaches the plane between the wires all 

 absorption has disappeared. 



In their general aspects these observations agree with 

 the measurements of the cross currents under a constant 

 E.M.F. between two test-electrodes at various points of the 

 discharge *. 



However, some differences still occur. 



In the first place, no variation of absorption in the different 

 parts of the striations was obtained with our electrodeless 

 method, while the electrode method yields a maximum of 

 conductivity at the bright parts of the striations. Again, 

 the maximum conductivity found by the electrode method is 

 at a point much nearer the cathode than that found by the 

 electrodeless method. The reason for the first discrepancy 

 is obviously that with the Lecher wires 42 mm. apart and a 

 tube diameter of only 18 mm. the electrodeless method must 

 yield, for the conductivity, values averaged over a bigger 

 distance than will be the case in the electrode method. The 

 second discrepancy can only be explained by a further in- 

 vestigation of this special point and will be left for the 

 moment. 



In order to concentrate the field between the parallel wires 

 more to a small region with the view of detecting the varia- 

 tion of absorption over the striae, we fixed a pair of wires, 



* See e. g. J. J. Thomson, Cond. of Electr. thr. Gases (2nd Ed.) p. 561. 



