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IX. — On the Balance Magnetometer, and its Temperature Corrections By J. A, 

 Broun, Esq. Communicated hy Sib T. M. Brisbane, Bart. 



(Read 21st April 18i5.) 



1. The Balance Magnetometer was imagined by Dr H. Lloyd, of Dublin, for 

 the purpose of observing the variations of the vertical component of the earth's 

 magnetic intensity. It consists simply of a balanced magnetic needle, with a 

 knife-edged axle, resting on agate planes, at right angles to the plane of the mag- 

 netic meridian. In the instrument from which the results in this paper are de- 

 duced, the position of the needle is observed by means of micrometer microscopes.* 



2. 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 

 of the centre of gravity from the centre of motion, t the angle contained by the 

 line joining these two centres, and the magnetic axis of the needle when hori- 

 zontal ; the equation of equilibrium will evidently be 



mY=Wgcose (1.) 



By differentiation and division 



— -=tan6Ae (2.) 



Y m ^ ' 



The differences a e are obtained by means of the micrometers, and the differences 

 of Y in terms of Y wiU be obtained, if we can determine e and , 'the latter 



being the variation of the magnetic moment, due to temperature. 



3. There are great practical difficulties in the way of rendering the needle 

 capable of giving e accurately by inversion, but Dr Lloyd has shewnf that 



tanf=cot2 0^' (3.) 



where 6 is the magnetic dip, T and T the times of one vibration of the balance 

 needle in a horizontal and in a vertical plane. We have thus, instead of one, three 

 unknown quantities to determine ; and it becomes a matter of importance to shew 

 with what degree of accuracy this may be done. 



The dip and time of vibration in a horizontal plane can be obtained with suf- 

 ficient truth for the purposes of this factor. 



* See the Introduction to the Malcerstoun Magnetical Observations for 1841-2. 

 t In his " Account of the Magnetical Observatory of Dublin," where the complete investigation 

 will be found. 



