96 
PROFESSOR R, THRELFALL AND MR. J. H. D. BREARLET 
sitiveness with ease and certainty in use, and in this we consider we have succeeded 
to an extent commensurate with our immediate wants. We do not see our way to 
the suggestion of any material simplification in design, without sacrificing convenience 
The voltage was supplied from the terminals by a two-tenths ohm coil, in series with 100,000 ohms 
and the Clark cell. The following are the readings :— 
Battery reversed. 
Battery reversed. 
Galvanometer reversed from A - B. 
Throw, 
18 divisions. 
99 99 
B- A. 
99 
16 
99 99 
A-B. 
99 
17 
9 ? 99 
B-A. 
99 
20 
99 99 
A-B. 
99 
19 
99 9 9 
B-A. 
9 9 
20 
Mean 
18*3 mici’ometer di 
Galvanometer reversed 
. . A-B. 
Throw 
, 27 divisions. 
99 9 9 
. . B-A. 
99 
24 „ 
9 9 9 9 
. . A-B. 
99 
24 „ 
9 9 9 9 
. . B-A. 
99 
26 
99 99 
. . A-B. 
,, 
23 
9 9 9 9 
. . B-A. 
9 9 
26 „ 
Mean 
25 divisions. 
Key in circuit only 
. . A-B. 
Throw 
, 0 division. 
9 9 99 
. . B-A. 
99 
1 „ 
99 99 
. . A-B. 
99 
0 „ 
99 9 9 
. . B-A. 
99 
0 „ 
Coil plugs inserted, so 
as to allow 
for resistance of terminals. 
Galvanometer reversed 
. . A-B. 
Throw, 2 divisions. 
9 9 9 9 
. B-A. 
99 
4 
99 99 
. A-B. 
9 * 
3-1 „ 
9 9 9 9 
. B-A. 
99 
2-5 „ 
9 9 9 9 
. A-B. 
9 9 
o 
• 9 99 
. B-A. 
99 
9 
^ 99 
9 9 9 9 
. . A-B. 
9 » 
2d „ 
99 9 9 
. . B-A. 
99 
3 
Mean 
2 3 divisions, say. 
Hence, mean deflexion for a resistance of 2 ohm between the points of derivation is 19'3 divisions. 
2 X 1-435 
Tlie current is less than 
10 13 
amperes, since the megohm Was in series with the galvanometer. 
The sensitiveness, therefore in amperes per scale division, is 
1-435 x 2 1-48 x 10~ 13 
1-93 x 10 13 ^ 
On a really steady day, 2 micrometer division can be read with absolute certainty ; hence, if this con¬ 
dition had obtained on this occasion, the sensitiveness for least observable deflexion would have been 
3 x 10 _i4 ampere, 
