394 Lord Blythswood and Dr. Marchant on 



With a current of 20 amps, (which did not heat the coils 

 excessively), a field of about 24.000 C.G.S. units was obtained 

 across an 8 mm. air-gap. When the air-gap was reduced to 

 1 mm., a field of 40,000 c.g.s. units was obtained. 



In these experiments, however, it was impossible to use a 

 gap of much less than 8 mm., on account of the size of the 

 capillary of the vacuum-tube used to obtain the mercury 

 spectrum. 



With the hollow pole-pieces the maximum field was about 

 16,000 c.g.s. units with an 8 mm. gap. 



The field-strengths were measured by means of a small coil 

 whose area had been carefully determined electrically, by 

 comparison with a standard single-layer coil wound on a core 

 of measured diameter. This coil was used in conjunction 

 with a ballistic galvanometer which had been standardized by 

 means of the discharge from a '5 m.f. condenser charged to 

 a known potential. The P.D. to which the condenser was 

 charged was measured by an Evershed voltmeter calibrated 

 by the potentiometer method with a Clark cell. 



§ 6. Results obtained of the Zeeman Effect on the Chief 

 Lines of the Mercury Spectrum. 



We may now consider some of the results obtained with this 

 instrument in the observation of the Zeeman effect on the 

 chief lines of the mercury spectrum. 



Attention was mainly directed to the blue (4358) line and 

 the green (5460), since these could both be photographed. 

 The blue line (fig. 7) photographed fairly easily, the exposure 

 necessary being from 10 to 20 minutes. The green line (fig. 1 0) 

 on the other hand, though very bright visually, generally 

 required about two hours' exposure. The other lines ex- 

 amined were the two yellow lines ; but no good photographs 

 were obtained. Measurements on them were made with a 

 micrometer-eyepiece. 



In photographing and observing these lines, it is very im- 

 portant that the tube should give a brilliant light. In order 

 to ensure this, fresh tubes were continually used, the exhaustion 

 being generally carried to 1 mm. or less. A drop of mercury 

 was put into the tube to give the requisite mercury-lines. The 

 tube was kept heated by a spirit-lamp placed underneath it ; 

 the lamp at the same time heated the iron of the pole-tips. 

 The alteration in the magnetic flux caused thereby was mea- 

 sured, and was found not to exceed 1 per cent. 



We may first consider the behaviour of the blue (4385) 

 line. Photographs are given, taken both in the single- and 

 double-order conditions. 



