720 Mr. H. Donaldson on the Spectra of the 



With the current 5*1 amperes the greatest sparking-distance 

 between the two spheres was found to be 11*2 centimetres. 



To sum up, the results given above show that the expression 

 (1) represents in its main features the wave of potential at 

 the terminals of the secondary coil when a current is inter- 

 rupted in the primary, and that it also gives the order of 

 magnitude of the maximum secondary potential. In each 

 of the cases described above some uncertainty with regard to 

 the maximum potential arises from the fact that the amplitude 

 of the oscillograph curve is not always the same with the 

 same primary current ; and although each curve was photo- 

 graphed a number of times and the curve of greatest amplitude 

 selected for measurement, we could never be quite certain 

 that the greatest possible amplitude was actually obtained. 



Similar differences are observed with other kinds of in- 

 terrupter. In Plate XIII. fig. 15, are shown the curves of 

 secondary potential at two successive interruptions of the 

 primary current produced by a motor mercury-interrupter 

 of the centrifugal type. The coils and condenser in action 

 at the time were the same as in Case VII. 



Bangor, July 1911. 



LXIX. On the Spectra of the Electrodeless Ring Discharge in 

 certain Gases. By H. Donaldson, B. A. (Camb.), B.Sc. 

 (Lond.), Scholar of Sidney Sussex College, ('ambridge*. 



IN an ordinary discharge tube with electrodes, the inter- 

 pretation of the differences in the spectra of a gas in 

 different parts of the tube is rendered difficult by the 

 separation which takes place between the positive and 

 negative ions. 



It was thought, therefore, that a study of the spectra of 

 gases in the oscillatory ring discharge, in which a permanent 

 separation between the ions would be unlikely, might afford 

 useful information concerning the dependence of the spectra 

 upon the strength of the ionizing field. 



Apparatus. 



The bulbs in which the discharge was obtained were usually 

 about 8 cm. in diameter and were surrounded by a coil of 

 half a dozen turns of well insulated wire wound closely so 

 that the depth of the coil was about 2 cm. The plane through 

 the centre of the coil parallel to the windings was also a 



* Communicated by Professor Sir J. J. Thomson. 



