Horizontal Component of the Earth's Magnetic Field, 489 



period of oscillation. The following is a description of the 

 arrangement and the mode of observation. 



A length of single cocoon-fibre was taken, and a stirrup 

 like that shown at S (fig. 2) formed on one end of it by fold- 

 ing it twice, so as to make four fibres, and making a knot 

 about 3 centim. from the end. The other end was passed 

 through a small hole in the brass bow, b (fig. 1), and then 

 fixed to a small strip of sheet lead which simply rested on the 

 sole plate and held the deflector in the proper position. The 

 deflector was placed in the stirrup S, and the loops adjusted 

 so as to suspend it horizontally. The magnet was thus sus- 

 pended in a stirrup almost devoid of inertia, and by a fibre of 

 negligible torsional rigidity. This leaves p,, the moment of 

 inertia of the deflector, and P, its period of oscillation, to be 

 determined. The moment of inertia can be very accurately 

 calculated from the mass, length, and thickness of the bar, 

 when, as is the case with these deflectors, the bar is a round 

 cylinder of small and perfectly uniform diameter. Greater 

 accuracy in these calculations, and the advantage of small 

 diameter in the " side on " position when taking deflections, are 

 the main reasons, apart from simplicity, why a solid cylinder 

 is preferred to a thin tube for the deflector. The period of 

 oscillation P was in the earlier experiments obtained either by 

 observing, with the eye placed behind a narrow slit, the times 

 of successive transits of the end of the bar across a fixed mark 

 (usually a black thread stretched vertically near the end of 

 the magnet and in the same magnetic meridian) ; or by ob- 

 serving the times of the successive transits across the vertical 

 wire of a telescope. In the later experiments, one end of the 

 deflector was polished with the view of using it as a mirror to 

 reflect a beam of light to a scale placed at some distance* 

 This method was not successful, owing to defects in the mirror, 

 and it has been abandoned in favour of a light silvered-glass 

 mirror, m (fig. 1), about 0'3 centim. in diameter and 0*01 

 gramme in weight, attached to the stirrup with its plane 

 parallel to the length of the magnet. The same lamp and scale 

 are thus available both for oscillation and deflection experi- 

 ments. With this arrangement the amplitude of the oscilla- 

 tion need never exceed one degree, and hence no correction 

 for arc is necessary. 



With regard to the effect of the inertia of the mirror on ^ 

 it is to be remarked that its total moment of inertia is about 

 •001, while that of the deflector is about 40 ; so that, even if 

 the inertia of the mirror be neglected, the error is not more 

 than ¥ ^o per cent. 



When a second observer was available, time was usually 



