C. HORIZONTAL INTENSITY. 



As already mentioned in the introduction, the magnetic apparatus 

 E. A. ZSCHAU No. 289 was arranged for the taking of both deflection obser- 

 vations at two different distances, and observations of the deflectors' time 

 of vibration, whereby the absolute value of the horizontal intensity may be 

 determined when the constants of the instrument are known. 

 If the following signs are employed: 



H = absolute horizontal intensity 



g> = angle of deflection 



e = distance of the deflector 



ft = a constant depending upon the distribution of magnetism in 

 the deflecting and deflected magnets 



fc' = induction coefficient 



K = moment of inertia of the magnet 



T = time of vibration, corrected for rate of chronometer, arc of 

 vibration and torsion force of the suspending thread. 



a = temperature coefficient 



ft = coefficient of dilatation for brass (O'OOOISO) 



ft* = coefficient of dilatation for steel (0'000124) 



I = temperature of magnet during the vibrations 



f = temperature of magnet during the deflections, 

 we have 



