Mr Almy, On a Diminution of the Potential Difference, etc. 187 



electric machine to furnish the discharge, so that the current could 

 be measured. A D'Arsonval galvanometer served as current-meter. 

 Potential measurements were made by the absolute (Kelvin) 

 electrometer. For the measurement of potential differences 

 several times as large as the range of the instrument, a very 

 high resistance, a long tube of xylol with several platinum 

 electrodes at points along the tube, was connected in shunt to the 

 discharge tube, and the potential drop over a part of this was 

 read off with the voltmeter. The relation of a part to the whole 

 was readily determined by observing the potential differences 

 between the electrodes of the different sections of the resistance, 

 with a constant total and summing the parts. 



The following series of observations, taken each with constant 

 current, show the character of the variation of the P.-D. between 

 the electrodes of a tube, with variation of cathode-area : 



Table V. 



Gas pressure 

 Current 



ce 



,0-12 





at 



0-28 



r± 13 



a 



0-78 



03 3 



•n ^ 



1-77 



"S ® J 



o 



2-54 



o rS 



-J 



3-80 



PH ^3 



O 



v 5-30 





I. 



II. 



III. 



IV. 



V. 



12 mm. 



12 



0-12 



0-08 



0*06 mm. 



1-0 



50 



60 



42 



50 



3000 



3600 







36000 



1500 



2300 



11200 



29400 





1300 



1800 



7000 



25200 



32300 



1000 



1300 



5600 



23800 



26400 



800 



1150 



6600 



22000 



30000 



700 



1000 



8700 



28600 



37000 



600 



800 



13000 



32200 



41000 



In the discharge of the Wimshurst, at low pressures, there 

 appears a noteworthy phenomenon. The character of the discharge 

 is of two distinctly different types. With cathode area small the 

 discharge takes place in streams, the discharge from the cathode 

 seeming to come off mainly from certain points or small spots, 

 at which a luminescence occurs, the discharge having a con- 

 siderable similarity to brush discharge from a point at atmo- 

 spheric pressure. Then, as the cathode area reaches a somewhat 

 larger value the discharge passes to the steady glow or " dark " 

 discharge common in the Crookes' tube at low pressure. 



So long as the first type of discharge takes place the potential 

 difference between the electrodes decreases as the cathode area 

 increases, but from the point at which the character of the dis- 



