242 Mr. S. S. Richardson on Magnetic Rotary 



falls and forms a very symmetrical button on the positive 

 and the arc remains in consequence very steady over long 

 periods. 



The tube to contain the liquids under test was about 

 1*5 cm. diameter, the ends being ground square and smooth. 

 To avoid contamination no cement was used, the end plates 

 being held on by placing between the tube-ends and the 

 poles two small rubber rings of the same diameter as the 

 tube. For the ultraviolet, plates of fused silica were at first 

 employed, but were found after a time to develop strains 

 introducing a small amount of elliptic polarization which 

 would reduce the sensitiveness of the apparatus. For the 

 experiments recorded below microscope cover-glasses *01 cm. 

 thick have been employed. These transmit quite well to 

 ♦275/^. 



The temperature was observed with a copper-constantan 

 thermocouple in series with a suspended-coil galvanometer, 

 preliminary experiments showing that the e.m.f. of this 

 couple was not affected by the magnetic field employed. 

 The cold junction was kept in ice. The tube containing the 

 liquid was enclosed in a cubical box at the bottom of which 

 was a small heater consisting of 50 cm. of No. 24 constantan 

 wound on a rectangular frame. This coil was joined in 

 series with an accumulator, amperemeter, and sliding rheo- 

 stat. It was thus found possible to maintain the tube at a 

 temperature of 20° C. within a few tenths of a degree. As 

 the heating effect in the magnet coils was scarcely per- 

 ceptible, the above method of maintaining the liquid at a 

 constant temperature was as a rule practicable for a 2 hours' 

 run. The small temperature variations allowed could be 

 neglected on account of the small effect of temperature on 

 magnetic rotation. 



The scale attached to the analyser mounting could be read 

 to 1', but most of the readings were taken on an accessory 

 " quartz-scale " (100 divisions = 21° 40') with vernier read- 

 ing to l'*3. The disposition of the apparatus shown in fig. 1 

 allows the settings to be made by the observer at the 

 spectrometer. In the visual region the value of the rotation 

 for each line was deduced by taking the mean from 12 

 settings of the analyser, the current being reversed after 

 each group of 6 settings. The extreme variation in the 

 readings was seldom more than 4' and with the brighter 

 lines much less than this. The values of 8 quoted in the 

 tables below are probably correct to the third significant 

 figure where the magnet was used, and to 1 part in 1000 

 with the solenoid. 



