730 



Dr. Thomas Martin Lowry 



[April V 



the lamp was raised into a vertical plane. The " end-on " lamp and 

 the vertical Quartzlite lamp have been descriljed in the Transactions 

 of the Faraday Society (1912, vol. YII. pp. 267-270), and were ex- 

 hibited at the Optical Convention of June 1912. The lamp which I 

 am using to-night has not been described previously. It combines the 

 merits of both of the preceding patterns, 

 _|- ^^ ' and can be used either horizontally or ver- 

 tically, and either in a side-on or in an 

 end-on position. 



The " Pinch Effect." 



One feature of the silica mercury-lamps 

 is sufficiently remarkable to deserve atten- 

 tion. When the arc is first struck by tilting 

 the lamp it fills the whole of the bore of the 

 half -inch tube which encloses it ; but, in 

 accordance with Faraday's observation that 

 currents travelling in the same direction 

 attract one another, the parallel threads of 

 current are drawn together until finally, as 

 you see, the arc is " pinched " together into 

 a thread occupying only about one-third of 

 the diameter of the tube. This pinching 

 together of the arc contributes substantially 

 to its efficiency as an illuminaut in polari- 

 metric and spectroscopic work, and enables 

 me to-night to use the arc as a linear source 

 of light without introducing any auxiliary 

 slit. The shape of the arc is, however, 

 altogether unsuitable for projection, which 

 demands, as a condition for successful work, 

 a powerful point-source of light. I must 

 therefore apologize for the small scale on 

 which I have been obliged to project my 

 illustrations, and should like to add that 

 even these imperfect experiments have only 

 been rendered possible by using the enor- 

 mous Nicol prisms bequeathed to the Royal Institution by Warren 

 de la Rue and by William Spottiswoode. 



If the current in the mercury arc is increased, the pinching effect 

 may extend to the point of breaking the threads of current completely 

 and so extinguish the arc. 



It may be of interest to some if I refer here to the well-known 

 fact that the " pinch efi'ect," which I have exhibited on a small scale 

 in one of these tiny mercury-lamps, is of great importance in the 

 electrical melting of steel on a large commercial scale. In that case 



Fig. 1. — Mercuey Lamp 

 foe use in a hoeizon- 



TAL OR VeETICAL SiDE- 



ON OE End-on Position. 



