ELECTRIC DISCHARGE WITH THE CHLORIDE OF SILVER BATTERY. 165 
one on the positive side, but the difference in the former case is far greater than in 
the latter ; in some cases there is little or no difference in the last but one and the 
last but two on the negative side ; in these cases the last but one on the negative 
side was dark,* while all the others had a luminosity about them. The difference 
of potential between the rest of the rings is sensibly uniform. 
The following observations, made December 21st, 1877, with tube 150, may be 
taken as an illustration of the method of measurement adopted. Batteries 6 and 7 
(2400 rod cells) were employed, and adjustable resistances were inserted in circuit 
for the double purpose of affording the means of readily varying the strength of 
current without interruption, and of enabling a measurement of that current to be 
made with the electrometer. The connexions are shown in the diagram, fig. 41. 
Fig. 41. 
The circuit was first broken by removing the earth wire from ring 17, and the plug 
T, in connexion with the induction plate I, being touched at any point between 
K' and ring 1 gave the reading for “Full Potential, open circuit;” next, the earth 
wire was replaced at ring 17, and the value for “Full Potential, closed circuit,” 
was obtained by causing T to touch at K'. As these batteries had but small 
internal resistance, the difference between these two readings was scarcely per- 
ceptible. By touching T at rings 1, 2, &c., in succession, their potentials were 
* An example of the dark space at conductors near the negative is shown at fig. 6, Plate 17. The dark 
spaces in a vacuum tube are regarded by De la Rive (‘ Comptes Rendus,’ lvi., 1863, pp. 672-673) as 
offering less obstruction to the discharge than the bright layers. He inserted in the tube discs of 
metal connected to wires passing through its walls ; these wires were connected to a galvanometer while 
the discharge was taking place : the derived current was found to be much less in the dark than in the 
bright portions of the discharge. 
