On a Complete Theory of the Bifilar Magnetometer. 443 



cessful as the inventor, Prof. Bell, and others anticipated. But 

 instead of this, the instrument will undoubtedly become of the 

 greatest assistance to physical research. In addition to sig- 

 nalling across rivers already referred to, Mr. Schwendler had 

 already used the telephone for indicating the speed of the in- 

 duction cylinder of a dynamoelectric machine (see precis of his 

 Electric Light Report); and Professor Hughes has used it 

 with great success in his induction-balance. It seems, in fact, 

 the telephone is too sensitive and too quick working for prac- 

 tical use. 



Calcutta, April 27th, 1880. 



LVIII. Complete Theory of the Bifilar Magnetometer and new 

 Methods for the Determination of the Absolute Horizontal 

 Intensity of the EartNs Magnetism, as well as of the Tempe- 

 rature and Induction-coefficients of Magnets. By H. Wild *. 



THE above is the title of a communication made to the 

 Imperial Academy of Sciences of St. Petersburg on the 

 15th (27th) January, 1880 (Bull. Acad. Imper. Sci. St. Peters- 

 burg, Melanges Phys. et Chim. xi.), as an abstract of a more de- 

 tailed paper to be shortly laid before the Academy. 



The starting-point of the paper is the observation that the 

 indications of the bifilar magnetometer, even when all causes 

 of disturbance are excluded, differ from the results of direct 

 measures of horizontal intensity to a far greater extent than 

 can be accounted for by errors of observation. The author 

 traces the cause of these discrepancies in the twisting of the 

 suspending fibres, of which no account is taken in the ordinary 

 theory of the bifilar magnetometer, according to which the 

 moment of the directive couple of the suspension is ascribed 

 simply to the weight of the magnet and its stirrup. The 

 effect which this cause is capable of producing may either be 

 calculated from the dimensions and coefficient of torsion-elas- 

 ticity of the fibres, or it may be determined experimentally 

 upon the bifilar instrument itself by observing the period of 

 vibration before and after a known change has been made in 

 the moment of inertia. Concordant determinations by both 

 methods indicated that in the case of a bifilar suspended by 

 brass wires 4 metres long, the couple due to torsion amounted 

 to 1 per cent, of that due to the suspended weight. In the 

 bifilar magnetometer of the St.-Petersburg magnetograph, 

 which was suspended by steel wires only 0*3 metre long, the 

 effect of torsion was as much as about 7 per cent, of the gra- 



* Communicated by the Physical Society 

 212 



