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IV. Experimental Researches on the Electric Discharge with the Chloride of 
Silver Battery. 
By Warren De La Rue, M.A., D.C.L., E.R.S., and Hugo W. Muller, Ph.D., F.R.S. 
Received August 7,—Read November 20, 1879. 
[Plates 8-10.] 
Part III.—TUBE-POTENTIAL; POTENTIAL AT A CONSTANT DISTANCE AND VARIOUS 
PRESSURES; NATURE AND PHENOMENA OF THE ELECTRIC ARC. 
Tube-potential. 
Dltring the course of the experiments described in Part lid* it could not fail to be 
noticed that the potential necessary to produce a discharge in partially exhausted 
tubes diminished with the pressure until a certain minimum pressure had been 
attained ; and that after this minimum had been reached, the potential had to be 
increased as the rarefaction was carried on, until at last 11,000 cells could no longer 
produce a discharge. Although Part II. contains many measurements from which it 
would be possible to calculate the tube-potential (the potential necessary to produce a 
discharge in a tube) for certain pressures, yet as they would not form a continuous 
series it was deemed desirable to make a special set of experiments with a constant 
number of cells, 11,000. This we have done in the case of hydrogen with a new 
tube, 162, 33 inches long and 2 inches in diameter, the distance between the ring 
and straight-wire terminals being 29'75 inches. In commencing each set of experi¬ 
ments the deflection of a tangent-galvanometer was observed when the battery was 
short-circuited; by a table previously calculated, the value of the deflection in ohms 
of resistance per cell could be read off; this multiplied by 11,000 gave the total 
resistance of the battery; the tube was then connected with the terminals and the 
galvanometer again observed, this gave a less deflection and indicated a greater 
resistance, which, multiplied by 11,000, gave the total resistance of the tube and 
battery ; by subtracting the resistance of the battery the resistance of the tube was 
ascertained. Calling the total resistance R, the tube-resistance r, the tube-potential V, 
Y= ' X . Thus in the first experiment cited, the battery, short-circuited, gave 
Ja 
a deflection of 61°, indicating a total battery-resistance of 230,000 ohms ; when the 
MDCCCLXXX. 
* Phil. Trans, for 1878, Part I. (Vol. 169). 
K 
