240 Mr. S. S. Richardson on Magnetic Rotary 



this wave-length were made both visually and photo- 

 graphically. 



The magnet available gave only a comparatively small 

 potential (5000 cm. -gauss with a 1 cm. gap), the polar faces 

 being 3 cm. square. Reduction of this by additional cones 

 was found not to increase the magnetic potential ; the set- 

 back of the main pole-pieces which their introduction 

 entailed counteracted the effect of concentration of the 

 field. The pole-pieces were bored with holes which, in order 

 to preserve the uniformity of the field, were only '25 inch 

 in diameter. Preliminary experiments both with an ex- 

 ploring coil and by magnetic rotation showed that the 

 magnetic potential increased with increasing interpolar 

 distance up to 4 cm. gap and then diminished. The tube 

 used for most of the liquids tested had a length 3*62 cm. 

 A current of 7*5 amperes was used which carried the 

 magnetization a little beyond the " knee " of the curve. ~No 

 advantage can be gained by the use of stronger currents, 

 since the small increase of field is obtained at the expense 

 of a rapid increase in the heating of the magnet coils. With 

 the above current the coils were only just warm to the touch 

 after a 2 hours' run. The current was obtained from a 

 battery of 50 accumulators, and as the resistance of the 

 magnet coils and battery leads was only 1*63 ohm, by the 

 use of a large ballasting resistance the current could be 

 maintained constant to 1 part in 750 for several hours at a 

 time. The ballast consisted of four open coils of bare 

 eureka 16 S.W.G., each 2 metres long and about 5 cm. 

 diameter. These were arranged in series with two parallel 

 wires of the same material and gauge, which, with a spring 

 brass connector, formed an exceedingly convenient and 

 reliable fine adjustment. The current was measured with a 

 millivolt-amperemeter by Siemens. 



A change in the field-strength may be produced through 

 the heating of the magnet cores. A careful ballistic test 

 showed, however, that with a current of 7*5 amperes the 

 error due to this cause would be quite negligible. 



The polarizing prisms were of the square-ended Foucault 

 type permitting complete polarization of a beam of small 

 angular diameter to about X *220 fi, and were supplied by 

 Hilger. A two-part polarizer was employed, the angle of 

 half-shade being adjustable. For most of the spectrum 

 lines selected an angle of 1°*5 could be used, but owing to 

 the narrow aperture in the pole-pieces and the consequent 

 reduction of the intensity of the light, it was not found to be 

 advisable to reduce the angle below this value. 



