177 
“On the Galvanometer” by William Jack, M.A., Pro- 
fessor of Natural Philosophy at Owens College. 
The author, referring to his communication entitled 
“Further Remarks on the Galvanometer,” printed in the 
last number of the Society’s Proceedings, points out that a 
serious misprint had occurred on page 159, where in the 
third line n-\-i should be n and i, He then suggests that 
the use of the formula he had given would afford a very 
satisfactory method of determining the magnetic length of 
213 
a bar magnet. Thus for we should have a negative 
result for any intensity between such as will make sin 2 ^ = 
105*3 
213 
and sin 2 ^ = 
116-7 
213 
Thus, if the very first entire length of 
magnet for which this reversal of effect takes place (trying 
L 2 
r* 
l 2, 212*1 L 2 
all possible intensities) be L, since and — 2 = a , 
a known fraction =r-„ must be > 
212-1 1 
and as I have 
L 2 315 a’ 
shown above That it is probably very possible to obtain by 
212-1 
sufficient care instead of the vague something which 
212*1 213 1} 
is at least between 0 -, g and 7^ can be determined with 
515 315 L 
2l3~T 
315 a' 
great accuracy and ~ will lie between J ^ and J 
Probably in practice much narrower limits might be found 
determinable, with ordinary values of i. Thus even for 
^=212-2, #=sin 2 a would lie between and ^ 1^2 which 
gives a fair range for a, so that it is unlikely that a good 
experimenter, carefully raising or lowering the intensities, 
would fail to observe the change in sign of deviation which 
corresponds. 
