110 
MESSRS. W. I)E LA RUE AND H. W. MULLER ON THE 
APPENDIX. 
Note A, relating to Page 69. 
In order to avoid tlie injurious running down of tlie life of the battery in making observations of the 
tube-potential, two new methods of observation were adopted subsequently to that described in pp. G5-68 ; 
one by means of the galvanometer, the other by means of the electrometer. The first method consists in 
observing, in the first place, the deflection, D, of a Thomson galvanometer through an adjustable resistance, 
so high in relation to the internal resistance of the battery that the latter might be neglected, then in 
observing the deflection, d, when the tube is connected with the terminals; the second deflection d, multi¬ 
plied by the number of cells employed, B, and divided by the first deflection, D, gives the tube-potential Y, 
D 
The resistance made use of is a line drawn with a hard (H) blacklead pencil along a V-shaped groove in 
a quadrangular prism of ebonite, one inch square in section, as shown in S S (fig. 88), half size. In the 
Fig. 88. 
T T' 
prism, on the under side, a recess is made not quite reaching the ends of the prism, and at the bottom of this 
recess the V groove is formed. A dovetail groove is also made, in which an ebonite cover slides, to com¬ 
pletely cover the recess so as to keep out dust. Into the cover two ebonite feet are screwed. Four brass 
rods, pierced partly through with holes to receive the terminals, are inserted through the upper surface so 
as to reach the apex of the V, and at the end of each rod, at the level of the apex, a small flat is formed 
which is rubbed over with blacklead to ensure contact with the pencil line. The length of the pencil line 
from A to B is 9T inches, B to C 9‘05 inches, and C to D 9'05 inches. 
The resistance of A to D was found to be 149,670 megohms. 
„ „ A „ C „ „ 50,789 „ 
», ,, A „ B „ „ 8098 „ 
