VARIATION APPARATUS. 6ll 



The reversal of the frame gives an inclination I 2 , corresponding 

 to a deflection in the opposite direction, and the mean of the de- 

 flections is 





The mean magnetic field of the deflecting needle on the de- 

 flected needle is proportional to its moment that is, C'M and 

 makes with the direction of the second needle a constant angle ; 

 the condition of equilibrium is 



(27) C'M sin = ' 



Comparing equations (26) and (27), we get 



_.:&/sin# cos I' C cos I' 



sin 8 sin (I - 1') sin 8 sin (I - I') 



We shall thus have the total force T, if the constant C has 

 been determined by comparison with absolute measures. 



1183. VARIATION APPARATUS. In permanent observatories 

 apparatus are arranged which give variations of the magnetic 

 elements, either by direct observation frequently repeated, or, 

 better, by continuous registration. It will be sufficient if we 

 indicate the general principles. 



Gambey's compass for variations of declination consists of a 

 magnetised bar 50 cm. in length suspended by silk threads, and 

 with a divided scale at each end which is read with a microscope. 

 The angular value of the division being determined, observation 

 gives the variation of the declination ^D, from a mark representing 

 a known declination. 



Long needles have the serious inconvenience that the oscillations 

 are too slow. Rapid changes may thus escape observation, and 

 considerable disturbances produce oscillations of great amplitude 

 which slowly become extinguished. It is better to take short 

 magnets and observe by the method of the mirror. 



1184. Several methods may be used for variations of the hori- 

 zontal component. 



The magnet of a declinometer being supported by a wire without 

 torsion, is permanently deflected by an auxiliary magnet M arranged 

 on a fixed post in the vicinity, so that the mean field of this bar 



R R 2 



