( «15 ) 



the true one, ttien our scale of magiietio forces is identical witli the 

 one commonly used. 



We may then deduce from a given separation of a well-detlned 

 spectral line the strength of a field in absolute measure, the constant 

 of reduction being once for all determined. 



In the measurements of Fariser ') relating to the lines 4678 Cd 

 and 4680 Zu (produced by a spark between zinc-cadmium electrodes) 

 the constant of reduclion could he determined with a probable error 

 of far less then Vioo- 



This method and all methods used till now for measuring 

 magnetic fields, give the intensity in a point. Or rather the mean 

 value in a small area (often rather extensi\e) or in a small space 

 is considered to be the intensity in a point of that ai'ea oi' of that 

 space. 



The magnetic separation of the spectral lines enables us to measure 

 simultaneously the macpietlc force in all points hehmfirui to a 

 straight line. 



In my experiments vacuum tubes charged with some mercury and 

 excited by a coil were used. The tubes had capillaries of 8 cm. 

 le'ngth, the inlerior diameters varying between V^ and '/i! '"'H- 

 The shape of the tubes was that given by Paschen ^), also used by 

 RuNGE and Paschkn in their investigation concerning the radiation of 

 mercury in the magnetic held. 



Avery moderate heating is required for the passage of the discharge, 

 the light in the capillary is then fairlj' intense, it becomes very 

 brilliant as soon as the tube is placed in the magnetic field. 



It was noticed that for a given vapour density there exists a 

 definite intensity of field for which the luminosity is a maximum. 

 This is easily seen when putting on the current of a dü Bois half ring 

 electromagnet. Owing to the large inductance (relaxation time 50") 

 the intensity of the field rises gradually. If the vapour density in 

 the tube is not too high, there is clearly one moment of maximum 

 luminosity. 



If with a given field the density of the vapour is well chosen, then 

 only a very moderate heating of the tube is sufficient for keeping 

 it luminous. 



When the tube is placed between the conical poles of a dü Bois 

 electromagnet and in a plane perpendicular to the line joining the 

 poles, there is of course a different field intensity in every point of 



1) Farber. 1. c. 



■-) Paschen, Eine Geisslersche Röhre zum Studium des ZEEsiAN-efl'ectes. Pliysik. 

 Zeitschr. p. 478. I. 1900. 



57* 



