700 



Mr. J. A. Cunningham on the 



raising and lowering of the mercury in the McLeod gauge 

 causing a movement of the heated and ionized air to and 

 fro between the discharge-tube and the P 2 5 bulb, and so 

 bringing about a more perfect drying which would, perhaps, 

 also partly explain the cathode-fall being so much greater 

 after the cathode had cooled down than it was before heating. 

 We have, however, already noticed a progressive increase of 

 this "lag" as the pressure was diminished. At this low 

 pressure (0*15 mm.) the whole curve is, as it were, tilted up 

 and there is only one real maximum value of the cathode-fall 

 a little below 1600° C. The previous maximum is here only 

 represented by a change of curvature. Above 1600° the 

 diminution of cathode-fall becomes very rapid. 



The potential-gradient in the rest of the tube (E— A) re- 

 mains practically unchanged with temperature. 



i Table XIV. 



Tempi of 

 Cathode. 



Potential Difference. 



Current. 



Pressui-e, 











mm of 



Defln. °C. 



i 



K-A. 



K-D. 



D-A. 



I)-B. 



Defln. 



Amps. 



xio 4 . 



Hg. 



2-7 103 i 





1460 



< 90 





104 



3-47 



0030 



2-8 



105 1 





1420 



55 





8-6 



2-87 





15-2 



430 j 

 545 



... i 1' 



55 











20-2 





1490 



„ 











26-2 



682 



... 



1460 



55 











26-8 



093 



... 



1420 



55 











49-9 



1158 





1400 



55 











04-2 



1410 





1470 













652 



1428 ; 



'.'.'. 1490 



55 





10-6 



353 





73-8 i 1573 



1560 













79-7 1670 



1490 



9) 





106 



353 





79-7 



1670 '■ 



1500 



55 





100 



333 





79-9 



1675 



1510 













80-4 



1683 



1510 



,, 











80-7 



1688 



1500 



100 









0-040 



800 



1676 



1420 



145 











80-8 



1690 





1410 



185 











80-4 



1683 , 





1380 



240 











80-4 



1683 





1385 



>310 



>310 



6-6 



2-20 





80-2 



1680 





1250 



90 



... 



17-8 



5-93 



0-052 



74-7 



1589 





1200 



55 



... 



194 



6-37 





66-9 



1458 





1140 







20-6 



6-87 





57-2 



1290 



... 



„ 



'.'.'. 



215 



7-17 





34-2 



850 





1080 



J5 



... 



222 



7-40 





32-2 809 





1030 



•5 











137 



395 





970 



„ 











11-2 



335 





960 



5J 











10-6 



320 



! 1030 940 



)( 





24-5 



817 



0061 



39 



140 



i 925 830 







11-6 



3-87 





31 



115 



| 890 



790 



JJ 





10 5 



3-50 





2-8 



105 



| S70 



1 



770 

 725 



220 





9-8 

 5-1 



327 

 1-70 





2-2 



85 



! 1030 



720 



>310 





0-8 



0-27 



0076 



