C. B. Bice — Beading Deflections of Galvcmometers. 277 



without loss. On the other hand, the disturbances of the field 

 being no greater than before would now have /i-fold less effect. 

 Hence, in an irregularly variable field such an instrument 

 would be ?i-time3 as accurate. In practice the gain is really 

 much greater than this because of the much reduced period of 

 the needle. These disturbances must be regarded as perfectly 

 irregular in the larger instrument, but may here be treated as 

 sudden occasional changes separated by periods of quiescence. 

 A galvanometer constructed on this theory, as developed by 

 Professor Hastings, has been tried with very satisfactory results. 



^^ — ^c^ 



The magnet is a circular steel disk of V"^ diameter polished 

 on both sides for convenience, serving as a mirror, hung be- 

 tween two coils of wire with a silk suspension of about S''"". 

 The coils are of number 36 wire with an outside diameter o f 

 2-5*™ and have a resistance each of six ohms. The brass covers 

 which fit over the coils are provided with openings closed by 

 glass so that there is a clear view of both faces of the magnet, 

 while air currents are excluded. A small aperture of 14°' "^ 

 diameter in the center of the steel mirror permits a view o f 

 objects beyond, which lie sufficiently near the axis of the instru - 

 ment. Facing the coil and the magnet within, but at a trifi e 

 higher level, is placed a small lens at a distance adjustable 

 from 5 to 7*^ from the magnet On the opposite side, at a 

 similar adjustable distance from the mirror, is fixed a fine 



