Bifilar or Horizontal Force Magnetometer. 
xxix 
§ 5. Bifilar or Horizontal Force Magnetometer. 
33. This instrument was made by Grubb of Dublin, and is similar, in its gene- 
ral construction, to the declinometer. An inner box was applied on September 26, 
1843 ; both boxes were gilt, externally and internally, and their joints covered with 
velvet. The magnet, whose dimensions are 15 inches, % inch and ^ inch, is placed 
in a stirrup, which carries below it a lens and glass scale connected by a tube ; the 
glass scale has 280 divisions, and the graduation at the 300th division, and increas- 
ing readings indicate increasing force ; the axle of a grooved wheel fits into the sus- 
pension eyes of the stirrup, the whole being borne by a silver wire passing round 
the grooved wheel, and having its two extremities fixed to a suspension roller ; the 
roller is supported by the torsion circle, which also bears beneath the roller a mi- 
crometer-headed screw, right-handed where it meets one wire (or portion of the wire), 
and leftr-handed where it meets the other. The screw serves to render the distance 
of the wires at the top equal to the distance at the grooved wheel. A copper ring 
encircles the magnet in order to check the vibrations. A thermometer by Ross, 
with a bulb 0*5 inch in diameter, is inclosed in a glass tube, and is fitted into the 
lid of the magnetometer box, leaving the bulb below, and the stem and scale above. 
This thermometer was intended to give the temperature of the magnet, but it was 
evident (especially while the box was imperfectly closed) that the temjDerature of 
the bar and of the air might differ considerably. In order to avoid this source of 
error, I had a thermometer made by Messrs Adie and Son, whose bulb rested in 
a cup, in a brass bar of the same dimensions as the magnet, and was covered loosely 
by a small brass cap. The following comparisons were made of the indications of 
the two thermometers, the box being in its original state, and the rise of the tem- 
perature considerable. From these comparisons, the necessity of some method 
which will give the temperature of the bar is at once obvious. As the thermometer 
by Adie is only partially in contact with the metal, it may be considered more as 
an indication of the temperature of the surface than of the interior of the bar. 
MAG. AND MET. OBS. 1843. 
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