109 



to its direction ; therefore at least in one of its causes must that sym- 

 metry-element lack too. Now in the above mentioned superposition, 

 conditions have become such as to make the occurrence of the 

 electric current possible: neither the electric field alone, nor the 

 magnetic field alone, can be the cause of the Hall-phenomenon 

 in a direction perpendicular to the plane of the current and the 

 magnetic lines of force ; but if both be combined, the symmetry-centre 

 of the magnetic field, as well as the symmetry-planes of the electric 

 field, - - with the exception of the single one just mentioned, - 

 will disappear, and now really all circumstances of symmetry (or 

 of Asymmetry) in the resulting cause will become such as to be 

 compatible with an eventual occurrence of the electric current (C ) , 

 as is observed in the Hall-effect. 



In the same way the motion of the string in Einthoven's string- 

 galvanometer, or that of the electric arc in the Birkeland-Eyde- 

 furnaces, will be symmetrical with respect to a plane passing through 

 the electrodes, or perpendicular to the lines of force of the mag- 

 netic field applied. 



However, the magnitude of such a predicted or expected effect 

 must be investigated in every case by special experiments ; as already 

 stated, it may be too insignificant to be detected by the usual 

 experimental methods. The general reasonings only teach us that, 

 if such an effect be produced by two superposed causes, the dis- 

 symmetries of the last are added to each other, and the symmetry 

 of the effect must be in accordance with the higher degree of dis- 

 symmetry (i. e. the lower symmetry) thus produced. 



The views concerning the symmetry or the dissymmetry of causes 

 and effects are principally identical : but in the one case is the chief 

 attention drawn to the symmetry-properties still present, in the 

 other case to the symmetry-properties which have disappeared. 



17. It may be remarked, moreover, that a number of problems 

 which can only be answered in the experimental way are connected 

 with the views developed in the preceding paragraphs. 



If only the superposed physical conditions be such that an effect 

 resulting from the interference of them may be expected with some 

 probability, it is worth trying such an experiment tentatively. 



Thus it may possibly be found that a difference of potential would 

 be observed with two electrodes plunged into a liquid of strong optical 

 rotatory power, if this fills a tube and be placed in a strong homo- 

 geneous magnetic field, having its lines of force parallel to the axis 



