THE SMITHSONIAN INSTITUTION. 



393 



Figs. 5 and 6 show plan and elevation of the bifilar compensator 

 two-thirds the actual size, a a, the magnet ; 6, the clamp, which 

 attaches the glass rod to the magnet; c c, the zinc tubes enclosing the 

 glass rod; d d, the adjusting clamps, consisting of two parts; the outer 

 encircles the zinc tube, the inner passes and nearly fills the interval 

 between the tube and glass rod. They are capable of sliding for 

 adjustment when the screws are loosened ; when tightened, the rod 

 and tubes are held together; e e, screws for adjusting the distance 

 between the hooks h h; these should be withdrawn when the clamps d 

 are fixed ; o o, fig. 5, are the ends of the clamping pieces interposed 

 between the tubes and the rod. 



The proper dimensions of 'the compensator and its approximate 

 adjustment are found by first ascertaining the temperature co-efficient 

 of the magnet, experimentally, from its time of vibration at high and 

 low temperatures, and calculating the corresponding proportions. 

 The more perfect adjustment is made after the instrument is com- 

 pletely mounted, by enclosing it in a box with a water-jacket, in which 

 the temperature of the water can be raised to any required tempera- 

 ture by heating a pipe connecting the inlet and outlet of the jackets, 

 and comparing the variations of the instrument at different tempera- 

 tures with the indications of another bifilar instrument, the temperature 

 of which has been maintained comparatively constant. 



VERTICAL FORCE INSTRUMENT OR BALANCED MAGNETOMETER. 



Fia.9 



Fig. 8 



