[ If ] 
II. On the Automatic Registration of Magnetometers, and Meteorological Instru- 
ments, by Photography. — No. IF. By Charles Brooke, M.B., F.R.S. 
Received May 8, — Read June 19, 1851. 
On the Automatic Temperature Compensation of the Force Magnetometers. 
A PORTION of the funds liberally contributed by the Government for the advance- 
ment of science, and placed at the disposal of the President and Council of the 
Royal Society, having been entrusted to the author for the accomplishment of the 
above object on a plan which was submitted to the Astronomer Royal and Colonel 
Sabine in the spring of last year*, and by them considered feasible, he considers that 
he cannot better fulfil the obligation of reporting progress at the present period, than 
by laying before the Royal Society a description of the instruments now constructed. 
So long as the results of the variations of magnetic force were deduced from eye- 
observation only, at the periods of which the temperature as well as the position of 
the magnets was recorded, a correction for the influence of change of temperature on 
the instruments themselves could be readily estimated and applied ; but in deducing 
mean values from the photographic registers, especially those for intervals involving 
considerable changes of temperature, it is manifest that the greatest degree of accu- 
racy cannot be attained, unless either the apparent values were individually corrected 
by means of a separate register of the thermometer enclosed in the box with the 
magnet, or the instrument possessed within itself an approximate automatic correction 
for the effects of change of temperature. 
The object would not be unattainable by the former means, but the process would 
be both difficult and laborious ; it therefore appeared more desirable to attempt its 
accomplishment by the latter. Referring therefore to the equation of equilibrium of 
the bifilar magnet, viz. 
wX=W^sin 6, 
in which m is the magnetic moment of the bar, X the horizontal component of the 
earth’s magnetic force, W the weight of the suspended bar and its appurtenances, I 
the length of the suspension skeins, a and h the upper and lower intervals of their 
centres, and & the angle of torsion, it is evident that the object in view would be 
* The author feels bound to express his belief that a somewhat similar plan of compensating the force mag- 
netometers subsequently proposed by Mr. Brown at the last meeting of the British Association was entirely 
original. ^ 
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