of the Bifilar Magnetometer, 445 



publication for the determination of the temperature-coeffi- 

 cients of magnets, and that he has assured himself by actual trial 

 of the precision of this method. 



The remainder of the paper is devoted to the description of 

 a process for determining in absolute measure the earth's hori- 

 zontal magnetic intensity by means of the bifilar magneto- 

 meter, and to the statement of the formulae required for the 

 complete reduction of the results so as to take account of all 

 corrections. The principles involved in this process may be thus 

 indicated: — It is divisible, like Gauss's process, into a method 

 for determining (a) the product of the horizontal intensity H 

 into the moment M of a particular magnet, and (b) the ratio 

 of the same two quantities. To determine the product, the 

 magnet is suspended in the bifilar instrument, and the torsion- 

 angle z is observed which is required to set the axis of the 

 magnet perpendicular to the meridian. This operation gives 

 the equation 



Dsin2=HM. 



The ratio of H to M is found by suspending another magnet 

 of moment M / in the bifilar, placing the first magnet with its 

 axis in the magnetic meridian through the centre of suspen- 

 sion of the instrument, and determining the torsion-angle z 1} 

 needed to set the suspended magnet perpendicular to the 

 meridian when the north-seeking pole of the first magnet 

 (moment M) is towards magnetic north, and also the corre- 

 sponding angle z 2 , when the north-seeking pole of the first 

 magnet is towards magnetic south. These observations give, 

 subject to the proper corrections, the equations 



2M> 



B f smz 2 =(R-^\W; 



from which the ratio in question is easily obtained in terms of 

 zi, z 2 , and the distance r between the centres of the mag- 

 nets. The horizontal intensity is then given by an expres- 

 sion which, when the corrections (for which we must refer to 

 the original paper) are omitted, becomes 



-p-2 __ 2D sin z sin z Y + sin z 2 

 r z sin &i— sin z 2 



