688 Mr. J. A. Cunningham on the 



temperatures were reached, it was often thought desirable to 

 vary the temperature backwards and forwards so as to try 

 and isolate the temperature effect as completely as possible. 

 It was also possible to raise the temperature of the platinum 

 grating by more gradual steps than would actually appear 

 from the numbers recorded in the accompanying tables. 

 Where in such cases a gradual change of temperature was 

 accompanied by a gradual and continuous change in the dis- 

 tribution of potential, it was often thought sufficient to record 

 the measurements of successive maxima and minima. And in 

 nearly all cases plenty of time was allowed to elapse for the 

 instruments to settle down to perfectly steady readings. 



A few general remarks on some of the appearances observed 

 may not, perhaps, be out of place here before proceeding to 

 a detailed record of the actual measurements made. 



The temperature of the cathode was observed to rise 

 gradually by the action of the discharge. At moderately 

 high pressures (0'5 to 2 mm.) and with small currents, on 

 first starting the discharge the negative glow was seen to 

 wander about in an unsteady manner over the surface of 

 the cathode, accompanied by a noise in the telephone which 

 only ceased after a very considerable lapse of time. This 

 unsettledness was most marked on starting the discharge for 

 the first time with a new wire. 



When the cathode was now gradually heated up the nega- 

 tive glow was observed to move away from the central hottest 

 portion of the grating and wander up into the side tubes, 

 vshere the wire was cooler. On further heating the discharge 

 would come back again and proceed from the hottest part of 

 the cathode. This phenomenon is consistent with the measure- 

 ments recorded below, which show a more or less well-marked 

 maximum "cathode fall'''' at temperatures below a yellow 

 heat varying with the pressure ; and it seems natural to 

 suppose that the discbarge will pass where it can do so with 

 the greatest ease. 



It will be seen at a glance from all the tables where the 

 cathode w r as taken through a cycle of temperature changes 

 that a sort of hysteresis becomes apparent. This may be 

 partly due to an error of observation. The cooling was 

 nearly always more rapid than the heating up. The method 

 of observation adopted was to keep an eye fixed on the volt- 

 meter after each successive reduction of current in the 

 primary of the transformer, and at successive readings of the 

 voltmeter to look up quickly at the corresponding reading 

 on the conveniently placed scale of the galvanometer con- 

 nected with the thermo-couple. Particular attention was 

 paid to recording maxima and minima on the voltmeter with 



