6lO MAGNETIC FIELD. 



the magnet deflected, provided that this distance is the same in 

 all observations. 



The oscillations of the deflecting bar give a quantity proportional 

 to the directing couple, and we may write 



MH = C'N 2 . 



The deflection observed by the method of sines, so that the 



bars are always in the same relative position, also give a quantity 



M 



proportional to the ratio : 



rl 



^ 



therefore 



C' N 2 



C" sin a 



The constant factor C is deduced from comparisons with ab- 

 solute measurements made at a principal station. The further 

 precaution is taken of determining the deflection a by the mean 

 of observations when the deflecting bar is inverted, and of chang- 

 ing side in respect of the deflected needle, in order to eliminate 

 the want of symmetry and of centring. 



1183. Dr. Lloyd* applied this method to the dipping needle. 

 Besides the needles A L and A 2 which serve for the ordinary ob- 

 servations, two special needles A 3 and A 4 are employed, the 

 magnetisation of which is never reversed. The former A 3 is well 

 balanced, the second A 4 is provided with a suitable counterpoise 

 which puts the centre of gravity outside the axis. 



The observation is first made with the last needle in the mag- 

 netic meridian. Let M be its magnetic moment, and I' the apparent 

 inclination; equation (17) gives 



(26) pd cos I' = TM sin (I - I'). 



This needle is then mounted almost at right angles and in a 

 position which is always the same in a limb which has micro- 

 scopes or lenses ; the needle A 3 is put in its place and its apparent 

 inclination I : observed. 



* Admiralty Manual of Scientific Inquiry, 4th Edition, p. 105. 1871. 



