44 ‘J. A. BROUN’S NOTE ON THE BIFILAR MAGNETOMETER. 
which turns the north end of the magnet from the north; the magnet will 
therefore move towards the north. The one action is in just the opposite direc- 
tion of the other, so that the conjoint action will be the difference of the two. 
Experiments with hot water on bifilar magnets have shown that a change 
of 1° Fahr. corresponds to from g = 0:00022 to g = 0°00038. If we take the 
mean of the two, the conjoint coefficient, including the action of temperature 
on the wires, should be 
gq = 000030 — 0:00011 = 0-00019 ; 
that is, the true coefficient is only about two-thirds of the coefficient by hot 
water experiments. This is nearly the mean of the results deduced from the 
determinations of the temperature coefficient by my method of comparisons 
‘and by the hot water method (see art. 39 of paper on the Bifilar Magneto- 
meter, previously cited). 
I have endeavoured to make this explanation of the cause of the very 
different results obtained by the two methods as clear as possible, since two of 
the most eminent magneticians, while accepting the facts, have expressed their 
inability to offer any explanation of them, though the cause here shown was 
suggested in my earlier papers on the subject (see Edmburgh Royal Society 
Transactions, vol. xvi. p. 77, art. 20; Makerstoun Observations for 1848; 
Edinburgh Transactions, vol. xvii. part 11. p. 53, art. 70). 
The conclusions arrived at for the unit and temperature coefficients have 
been confirmed in every instance in which the two methods for each coefficient 
have been tried; and it may safely be asserted, that if in any case this con- 
firmation is not obtained, this will be due to some error in the determinations. 
I believe that we have at present no more perfect magnetical instrument 
than the bifilar magnetometer, when proper precautions have been taken in its 
construction, and when the coefficients have been determined by the methods 
referred to in this note. It may be remarked, that the wires should be of 
gold (,% fine is probably the best), suspended by tubular pincers, and not 
wound ona roller. It is however, also, of the greatest importance that the 
true temperature of the magnet should be observed, and this can be got only 
when the thermometer bulb (resting on a thin metal plate) is within the inner 
box near the magnet. Every precaution also should be taken to make the 
changes of temperature as small as possible. 
These precautions are essential if a scientific instrument is required. Bifilars 
with silk suspension threads are not scientific instruments ; they are affected 
by humidity and by unceasing changes in the strain and disposition of the 
thread fibres, as well as by their frequent rupture, for all of which sources of 
error no correction is possible. 
