82 Mr Purvis, The influence of very strong 



The influence of very strong electro-magnetic fields on the 

 spark spectra of gold, antimony, bismuth, lead, and tin. By 

 J. E. Purvis, M.A., St John's College, Cambridge. 



[Read 27 February 1905.] 



The aim of this preliminary paper is to describe the effects on 

 the appearance of the spectra of the above-mentioned elements, 

 when their spark discharge takes place in very strong electro- 

 magnetic fields. 



Description of the apparatus. 



The spectroscope used in the experiments is Professor Liveing's 

 large 21-foot concave grating. The spectrum of the second order 

 was used for wave-lengths from about A, 3600 and extending into 

 the ultra-violet ; and that of the first order for lines whose wave- 

 lengths are below X3600. The width of the slit was varied. from 

 time to time, when it was required to examine the effect on the 

 stronger or weaker lines. In the case of very strong wide lines, 

 the slit must be considerably narrower than for finer lines, if the 

 ' Zeemann' effect is to be seen. 



Self-induction coils were introduced occasionally : and al- 

 though, as has been before noticed, the lines appear to become 

 sharper, there is the drawback that they appear to be weakened 

 as well, and also the weaker lines become so much weakened as 

 to have no effect on the photographic plate. 



The electro-magnet is of the du Bois form. The poles are 

 conical, and the noses of the pole pieces are 7 mm. diameter. 

 In these experiments the distance between the poles was 4 mm. 

 In this space, with a current of 25 amperes, a field strength of 

 about 41,000 C.G.s. units can be obtained. A current of 21 amperes 

 was employed in these experiments and this gives a field of 

 40,000 units. The coils wound round the magnet become warm 

 after 40 minutes running, and when larger exposures were necessary 

 to photograph the weaker lines, it was necessary to cool down, 

 and repeat the experiment after the magnet had cooled. This 

 did not seem to interfere with the exactness of the definition 

 of the lines, unless there was an excessively windy day. 



The current supplying the magnet was obtained directly from 

 a dynamo. It was easily controlled when allowing for the in- 

 creased resistance as the coils became warmer, and there was 

 no difficulty in keeping the current constant as shown by the 



