IV. Experimental researches on the electric discharge with the chloride of silver battery

Abstract

Tube-potential . During the course of the experiments described in Part II. 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 experiments 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, V= r × 11,000/R. Thus in the first experiment cited, the battery, short-circuited, gave a deflection of 61°, indicating a total battery-resistance of 230,000 ohms; when the tube was connected a deflection of 7°, indicating a total resistance, tube and battery, of 3,383,000 ohms = R, 3,383,000—230,000=3,153,000= r the tube resistance, 3,153,000 x 11,000/3,383,000= 10,250=V. The deflection observed was found to be different when the ring was made positive and negative respectively; the following results were obtained in each particular case:— Observations July 3rd, 1878. — 11,000 cells, short-circuit, deflection 61°, total battery-resistance = 230,000 ohms; deflections afterwards taken with the tube connected, the ring being positive.