ELECTRIC DISCHARGE WITH THE CHLORIDE OF SILVER BATTERY. 489 
inserted resistance of 200,000 ohms, a current of (P00335 ampere, which produced a 
perfectly steady stratification during more than thirty-five minutes the tube was in 
connexion with tire battery. In two and a-lralf seconds an impression of the strata was 
obtained on a dry plate, which is represented in Plate 36, fig. 21. The strata were then 
carefully covered so as to exclude all their light from the camera and to expose only the 
dark space; in fifteen minutes an impression was produced on the photographic plate, 
but it required an exposure of thirty-five minutes to produce an impression as dense 
as that obtained by an exposure of the strata for two and a-half seconds. The photo¬ 
graphic intensity of the dark space is therefore 840 times less than that of the strata. 
The image of the dark space is shown in Plate 36, fig. 21, below that of the strata; it 
contains no trace of stratification, although, if any existed, it is fair to presume that it 
would have been depicted in consequence of the perfect steadiness of the discharge. 
Potentials at different nearly aliquot parts of a column of gas, at various pressures. 
The following experiments, intended to ascertain the difference of potential in 
different parts of a vacuum tube,* * * § bring out instructive information, not only in 
reference to the relative resistances of different lengths of a column of gas at various 
pressures, but also in regard of the impediment presented by the terminals themselves 
to the passage of a discharge from gas to terminal or terminal to gas. 
They were made by means of an electrometer in connexion with an induction 
apparatus shown in fig. 13, and described in Part III. of our researches.! 
In the present instance, however, certain modifications were adopted to prevent 
disturbing influences and to ensure concordant results. In the first place, a trap was 
formed to prevent the creeping clown of electricity on the inside and outside, more 
especially the outside of the cylindrical glass shade enclosing the induction plates. 
This trap consists of two hoops of tin foil,J half an inch in width, pasted inside and 
outside respectively, about 4 inches below the flat top. Both these hoops communicate 
to earth by means of two vertical strips of tin foil pasted on the cylinder. Before 
this arrangement was adopted the cylinder became slowly charged, especially in damp 
weather, and unduly increased the deflection of the needle of the electrometer, by 
acting inductively on the induction plates. 
It was found very advantageous to connect the needle with a small Leyden-jar§ 
(L, fig. 14), presenting a coated surface of 57 square inches (3'68 square decims.), and 
having a capacity of 0'0016 microfarad. The jar, by increasing the capacity of the 
apparatus, in a great measure obviates the disturbing influence of leakage during the 
* Tube 149 with 12 aluminium rings, and tube 150 with 17 rings already described. Phil. Trans, 
for 1878 (vol. 169), p. 165 ; Part II. (separate copy, p. 81). 
t Pbil. Trans, for 1880, vol. 171, p. 112 (separate copy, p. 206). 
f These are not shown in the engraving. 
§ Part I. of our researches, Phil. Trans, for 1878, vol. 169, p. 99 (separate copy, p. 45). 
