10 . REPORT—1850. 
tained when a proper length of brass rod is employed. In the case of Sir Thomas 
Brisbane’s bifilar Am for 1° Fahr.=0-000266, the interval of the wires is nearly half 
= : 
an inch, and therefore the brass rods would require to be each about 73 inches long, 
in order that the interval be diminished 0°000266 of itself (the coefficient of con- 
traction), or 0°000133 inch, the difference of the coefficients of expansion of brass 
and wood being assumed equal to 0°0000085. Magnets with a temperature coeffi- 
cient of 0°0001 would require brass rods of 3 inches in length, or smaller, as the 
interval of the wires is less than half an inch. 
I propose the following process for the compensation of the balance magnet :— 
Let a brass rod be fixed to the magnet near its south end, but free to expand towards 
the north, and having its centre of gravity near the centre of motion; it is obvious 
that when the temperature increases, the north end of the magnet rises, from the 
diminution of its magnetic moment, but at the same time the expansion of the 
brass rod towards the north end will tend to depress it; by a proper regulation 
therefore of the length and weight of this brass rod (which will depend upon the 
weight of the needle and the distance of the centre of gravity from the centre of 
motion), the two effects of temperature may be made to destroy each other. For 
An 
the Makerstoun balance, for which —=0°00008, I have computed that a brass rod 
m 
10 inches long, one-thirtieth the weight of the needle, placed as has been proposed, 
would compensate nearly for the variation of the magnetic moment. 
In both cases such computation could only be considered as guides to the instru- 
ment-maker, who, by experimeats at different temperatures, might be able to attain 
a very accurate compensation. 
These compensations, it is conceived, will be most useful, especially for self-re- 
gistering apparatuses. For other instruments, should the compensation not be quite 
perfect, while it would serve for all large yariations, it might be insufficient for more 
delicate investigations ; for these, however, the residual temperature coefficient could 
be obtained from the observations themselves, by the process which I have adopted 
in the correction of the Makerstoun observations. ] eye 
On the Construction of Silk Suspension Threads for the Declination 
Magnetometer. By J. A. Broun, F.R.SL. 
Till the year 1777 the magnetic declination was observed by means of a magnetic 
needle balanced upon a steel pivot, as in the common mariner’s compass ; the amount 
of friction in this mode of placing the needle rendered it unfit for any delicate in- 
vestigation, and the French Academy of Sciences, which had observed this deficiency, 
proposed the improyement of the suspension as the subject of a prize. Coulomb, who 
wrote one of the papers crowned, proposed in 1777 suspension by means of a thread 
formed of the silk fibres from the cocoon. This suspension was adopted immediately 
afterwards by Domenic Cassini, although the cup and pivot were used by others, as 
by Gilpin, in the present century. The importance of the subject will be easily 
» understood when it is remembered that the labour of years and one of the chief ob- 
jects in the formation of magnetic observatories, may be frustrated by a bad sus- 
pension thread. 
The suspension thread acts in the following manner :—As the thread is composed 
of a series of fibres more or less twisted, the plane of detorsion, that is the vertical 
plane in which an unmagnetic bar will rest when suspended, is determined by the 
composition of a series of opposing forces: if the torsion of the individual fibres be 
at all considerable, very small motions of the magnet will cause them ta occupy 
slightly different positions, or moderate changes of humidity acting to a greater ex- 
tent upon the external than the internal fibres, and upon some of the external fibres 
more than upon others, will change the plane of equilibrium, and in this way force 
the magnet from its true position. ~ 
Cassini, in order to avoid these sources of error, formed his thread in the follow- 
ing manner :—Haying cut the fibre into proper lengths, he stretched them singly 
by means of weights; he then joined them together and passed the thread thus 
