128 OH Energy in the Electromagnetic Field. [June 18, 



to a working stress. Like the Poynting flux, it contains vector pro- 

 ducts. From this flux the stress itself is derived, and the form of 

 translational force, previously tentatively developed, is verified. It is 

 assumed that the medium in its motion carries its properties with it 

 unchanged. 



A side matter which is discussed is the proper measure of " true " 

 electric current, in accordance with the continuity of energy. It has 

 a four-fold make-up, viz., the conduction current, displacement cur- 

 rent, convection current (or moving electrification), and the curl of 

 the motional magnetic force. 



The stress is divisible into an electric and a magnetic stress. These 

 are of the rotational type in eolotropic media. They do not agres 

 with Maxwell's general stresses, though they work down to them in 

 an isotropic homogeneous stationary medium not intrinsically mag- 

 netised or electrised, being then the well-known tensions in certain 

 lines with equal lateral pressures. 



Another and shorter derivation of the stress is then given, guided 

 by the previous, without developing the expression for the flux of 

 energy. Variations of the properties permittivity and inductivity 

 with the strain can be allowed for. An investigation by Professor 

 H. Hertz is referred to. His stress is not agreed with, and it is 

 pointed out that the assumption by which it is obtained is equivalent 

 to the existence of isotropy, so that its generality is destroyed. The 

 obvious validity of the assumption on which the distortional activity 

 of the stress is calculated is also questioned. 



Another form of the stress vector is examined, showing its relation 

 to the fictitious electrification and magnetic current, magnetificatioii 

 and electric current, produced on the boundary of a region by termi- 

 nating the stress thereupon ; and its relation to the theory of action 

 at a distance between the respective matters and currents. 



The stress subject is then considered statically. The problem is 

 now perfectly indeterminate, in the absence of a complete experi- 

 mental knowledge of the strains set up in bodies under electric and 

 magnetic influence. Only the stress in the air outside magnets and 

 conductors can be considered known. Any stress within them may 

 be superadded, without any difference being made in the resultant 

 forces and torques. Several stress formulae are given, showing a 

 transition from one extreme form to another. A simple example is 

 worked out to illustrate the different ways in which Maxwell's stress 

 and others explain the mechanical actions. Maxwell's stress, which 

 involves a translational force on magnetised matter (even when only 

 inductively magnetised), merely because it is magnetised, leads to a 

 very complicated and unnatural way of explanation. It is argued, 

 independently, that no stress formula should be allowed which indi- 

 cates a translational force of the kind just mentioned. 



