Balance or Vertical Force Magnetometer. 
xxxv 
and Son resting on a brass bar. From September 26 d till November 27 d the tem- 
perature of the magnet was probably worse determined than at any other time, as 
the bulb of the thermometer by Ross was outside of the inner box, while the magnet 
was within it. December 14 d l h , the inner box was removed in order to compare 
the indications of the thermometers of Ross and Adie ; it was not replaced till 
January 10 4 3 h 1844. 
§ 6. Balance or Vertical Force Magnetometer. 
47. The balance magnetometer was made by Robinson of London. It is com- 
posed of a needle 12 inches long, and about f inch broad, with knife-edged axle 
resting on agate planes ; at the extremities of the needle are brass rings, each carry- 
ing a cross of spider threads. The needle is placed horizontally at right angles to 
the magnetic meridian ; it is accurately adjusted by means of two fine brass screws ; 
one towards one extremity, working horizontally, balances the needle ; the other, 
working vertically near the other extremity, regulates its sensibility. The appa- 
ratus is covered by a rectangular box, having glazed openings on two sides opposite 
the spider's crosses ; those on one side allowing light to be thrown on the crosses 
from two small mirrors ; those on the other for viewing them and determining their 
position, which is done accurately by means of microscopes carrying micrometers. 
A thermometer within the box gives the temperature of the needle. 
48. If m be the moment of free magnetism of the needle, Y the vertical com- 
ponent of the earth's magnetic force, W the weight of the needle, g the distance 
from the centre of motion to the centre of gravity, € the angle contained by the line 
joining these two centres and the magnetic axis of the needle, the latter being hori- 
zontal, the equation of equilibrium is 
»Y = ¥j cos e 
differentiating this equation, dividing by it, and having regard to the sign of a €, 
we have 
A Y Am 
^ T = tan € A € 
x m 
the varying angle which the magnetic axis makes with the horizontal, a e, is ob- 
tained from the micrometer observations. See the section on the temperature cor- 
, • <• Am 
recti ons tor . 
m 
49. It is conceived that € cannot be determined with accuracy by the method 
of inversion, owing to mechanical difficulties in the formation of the axle, but Dr 
Lloyd has shewn* that if the needle be disturbed through a small angle n, the mo- 
ment of the moving force brought into play by the disturbance is equal and opposite 
* Account of the Magnetical Observatory of Dublin, p. 38. 
