ON THE SPECIFIC RESISTANCE OF MERCURY. 
170 
general repeated between each refilling with mercury, but it was omitted in measure¬ 
ment 7, and there is no record of its having been done in 1, 3, and G. 
To calibrate the tubes a short thread of mercury was inserted and moved to the 
various positions required, by blowing through a chloride of calcium tube. In the case 
of tubes I. and II,, the length, A, of the thread was measured by adjusting micro¬ 
scopes to its two ends, with subsequent substitution of an ivory scale divided in 
fiftieths of an inch. But this method was troublesome; and with tubes III. and IY. 
the scale was simply placed against the thread and the length read off with a 
magnifying-glass, a procedure which was found to give sufficiently accurate results, 
notwithstanding the difficulty arising from parallax owing to the thickness of the glass. 
The following table gives the different values of A for each tube. 
As a check upon the correction for conicality, two distinct values of [x were in some 
cases calculated from the alternate observations of A, and were found to agree closely. 
It may not be superfluous to mention that in carrying out the computations we 
must work to six or seven places, although the observed values of A themselves may 
not be accurate beyond the third place. 
The lengths are in fiftieths of an inch. 
Tube I. 
Tube II. 
Tube III. 
Tube IV. 
80-8 
104-5 
135-0 
171-0 
80-0 
104-1 
134-0 
172-0 
77-0 
104-5 
133-0 
171-5 
75-8 
105-0 
132-0 
170-5 
76-0 
104-5 
131-5 
171-5 
76-4 
105-2 
130-5 
174-5 
75'0 
104-3 
128-0 
175-0 
74-0 
104-0 
127-5 
174-5 
73-4 
104-7 
126-5 
175-5 
73-0 
104-0 
126-5 
176-5 
72-7 
103-0 
126-5 
177-0 
72-3 
101-8 
126-0 
180-0 
72-5 
125-0 
180-5 
71-9 
125-5 
180-7 
71-1 
126-0 
182-2 
70T 
126-0 
183-7 
69-7 
126-0 
183-5 
68-0 
126-5 
182-5 
67-9 
127-0 
184-0 
67-G 
127-0 
186-0 
65-9 
128-5 
186-5 
65-3 
128-0 
128-5 
128-0 
To find the mean section of the tubes we at first tried the method adopted by 
Messrs. Matthiessen and Hockin in their experiments for the British Association. 
After aspirating the tube with dry air we placed it in a wooden trough full of mer- 
2 A 2 
