Magnetic Separations of tlio Linos of Iron, Nickel and Zinc in Different Fields. 5 



2. Electromagnet. The magnetic fiele, in which the light 

 source was placed, was excited by an electromagnet of Du-Bois 

 half-ring type. Various fields were obtained by changing either 

 the pole-gaps or the magnetizing current or both. I used four 

 kinds of iron tips: 4 mm., 5'5 mm., G mm. and 0'5 mm. in 

 end diameters according to the kind of spark desired as 

 well as the width of the magnetic gap used. The length of the 

 spark was always smaller than the diameter of the pole tips, and it 

 was always placed in uniform fields. The gaps l:)etween the poles 

 Averel'omm., 2"0mm., 2"5mm., 3*1 mm., 3'7mm, and 4"0 mm. 

 The magnetizing current varied from 2 to 13 amperes. The 

 vertical angle of the conical pole-pieces was about 120" to attain the 

 maximum field. The highest magnetic field thus attained was 

 34120 gauss. The electromagnet can be excited for as long as six 

 hours at 10 amperes, but it must be cooled down after three 

 continuous hours when a current of 13 amperes is used. The 

 current was obtained from the secondar}^ batteries in the laboratory. 

 It was easily controlled when allowance was made for the 

 increased resistance as the coil became warmer, and there was no 

 difficulty in keeping the current constant as shown by the 

 ammeter. The magnet was arranged broadside on, that is, the 

 lines of force between the poles were in a direction perpendicular 

 to the line joining the slit and the grating. 



3. Spectrograph. The spectroscope used in this investigation 

 was a Rowland concave grating whose focal length is 10| feet, the 

 breadth and the height of the ruled surface are 3i and 1| inches 

 respectively. Tlie number of ruled lines per inch is 14,438. 

 The resolving power -g— ^i^^^ the linear distance in mm. on the 

 photographic plate, corresponding to a change of wave-length of 



o 



one Angstrom unit, was calculated by tlie formulae inserted in 

 Baly's "Spectroscopy" (p. 171, 1905). Let ;. be the wave-length 

 in A. U., and g the linear distance in mm. from the spectral line 

 of wave-length À to the slit, then we have the following data for the. 

 ^grating: 



