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XII. Experimental Researches on the Electric Discharge with the Chloride of 
Silver Battery. —Part IV. 
By Warren De La Hue, M.A., D.C.L., Bh.D., F.R.S., and Hugo W. Muller, 
Ph.D., F.R.S: 
Received June 11,—Read June 14, 1883. 
[Plates 35, 36.] 
TUBE-POTENTIAL. 
Pressure of least resistance dependent on the shape and dimensions of vessel. 
We have already stated that the potential necessary to produce a discharge in partially 
exhausted tubes diminishes with the pressure until a certain pressure of minimum 
resistance has been reached, but as the rarefaction is increased beyond this point, then 
the potential has to be increased in order to produce a discharge/'" The experiments 
on which this result was founded were made with a tube 33 inches (83'8 centims.) 
long and 2 inches (5 T centims.) in diameter, and it was found that in a hydrogen partial 
vacuum the pressure of minimum resistance was 0'642 m.m., 845 M. Professor 
Stokes suggested the desirability of making experiments with a wide tube, or, still 
better, with a globe, as it would allow the discharge to spread laterally and diminish 
the resistance thereby, and, very probably, alter the pressure of minimum resistance. 
He considers this to be a question of importance with reference to the height ol the 
aurora, for in the atmosphere there is ample space for lateral expansion, and it is 
conceivable that the least resistance may correspond to a pressure a good deal different 
from that which gives the least resistance in a tube. 
Experiments were made in an air residue contained in a vessel in some measure 
resembling a prolate spheroid, 7 inches (17"8 centims.) long and 5 inches (12 v centims.) 
in diameter, the distance between the terminals, one a cup, positive, the other heart- 
shaped, negative, was 3f inches (9'2 centims.) (see fig. 1). The battery employed 
Fzy.l 
* Part III., Phil. Trans, for 1880, vol. 171, Part I., p. 65 (separate copy, p. 159). 
MDCCCLXXXIII. 3 Q 
