6l2 MAGNETIC FIELD. 



on the declinometer is very nearly perpendicular to the magnetic 

 meridian. 



Suppose, generally, that the direction of the field F makes an 

 angle a with the perpendicular to the meridian, and the angle 

 with the direction of the deflected magnet. The condition of 

 equilibrium is 



H cos(0-ta) = Fsin<9. 



When the horizontal component varies by </H and the declina- 

 tion by dD = da, these variations being very small, the corresponding 

 deflection dd of the declinometer satisfies equation 



H. 



If the defect of adjustment a is very small and the angle is 

 near 45, as is usually the case, we may write 



Jti r 



dD being given by the variation apparatus. 



The force F changes with the temperature and with the time. 

 If F is its value at a certain epoch and the temperature 0, we 

 may represent it at the temperature /, and at the time T, by an 

 expression of the form 



F = F (i -*/)(i -JT) = F (i-/-*T); 

 it follows that 



(28) = -at-t>T + 2dO + dD. 



H 



The greatest difficulty is to know the coefficient b of gradual 

 enfeeblement. It is necessary often to compare the indications 

 of the apparatus with the results given by the direct determinations 

 of H. 



1185. Another method consists in supporting a bar by a bifilar 

 suspension, with a torsion such that the bar is almost in the trans- 

 verse position. 



Let be the angle of the bar with the direction it would take 

 if it were not magnetised, and therefore the torsion of the bifilar, 



