100 Scientific Intelligence. 



the smaller glass tube. The diameters of the metallic cylinders 

 were 8 # 20 cms and l-27 cms respectively, with a common length of 

 18'0 cms . No guard-ring system was employed, so that the end 

 correction was not eliminated. However, this correction was 

 unimportant under the experimental conditions. Details of the 

 electrical connections and of the arrangements for varying the 

 gas pressure will be omitted in this place. The order of an 

 experiment was to keep the pressure constant while taking a 

 series of readings of length of dark space and voltage for four or 

 five definite current values, and then to change the pressure and 

 make a new series of observations. 



The general features of the discharge were the same as for 

 plane electrodes. With oxygen the edge of the dark space was 

 absolutely sharp at all pressures, whereas with hydrogen this 

 edge was not nearly so well defined. When the experimental 

 data are plotted with the lengths of the dark space as abscissae 

 and with the corresponding reciprocals of the square root of the 

 current densities as ordinates, parallel straight lines are obtained. 

 The "observed " points fall close to the mean lines both when 

 the outer cylinder and when the inner one had been used as 

 cathode. Not only is the slope coefficient B, of equation (I), con- 

 stant, but it has the same value as for plane electrodes of alumin- 

 ium. The only proviso is that the current in each case must be 

 expressed as current density at the surface of the cathode. This 

 independence of the curvature of the electrodes is particularly 

 surprising when the fact is taken into account that the radius of 

 curvature was of the same order as the length of the dark space, 

 so that the current density at the edge of the negative glow must 

 have been widely different from that at the surface of the 

 cathode, and its distribution continually altering with the change 

 in the length of the dark space. The equality of B for plane 

 and cylindrical electrodes is a very important result, and it seems 

 to indicate that, whatever may be the explanation of the shrink- 

 age of the dark space with increase of current density, the 

 mechanism which causes it is probably at or very near the surface 

 of the cathode. Since B is constant, for a given gas, the observed 

 changes in the pressure factor A lead to the conclusion that the 

 effect of curvature of the cathode on the length of the dark space 

 is exactly like a change in pressure, an increase if it is concave, 

 and a decrease if it is convex. Equation (II) was also found to 

 hold both for hydrogen and oxygen when coaxial cylinders were 

 used as electrodes. — Proc. Hoy. Soc, vol. lxxxvii, No. A 597, 

 p. 428, October, 1912. h. s. u. 



8. On the Influence of the Nature of the Cathode on the 

 Length of the Crookes Dark Space. — This investigation, by F. 

 W. Aston, followed as a natural consequence of the results 

 briefly described above. Only parallel plane electrodes in the 

 gases hydrogen and oxygen were employed. The pairs of elec- 

 trodes were made of aluminium, iron, copper, zinc, silver, tin, 

 platinum, and lead. A few experiments were performed with 



