of Emission and Absorption Lines in a Gas-Spectrum. 271 

 equal to ( - j multiplied into the total electric flow through 



its aperture. 



\c denotes the velocity o£ radiation, and is introduced 

 conformably to the units adopted.] 



The line integral o£ magnetic force means that we take 

 the tangential component of the force, multiply it by the 

 element Bs of length of the circuit, and then add up all round. 



The above statement of the first relation could have no 

 meaning unless the electric flow were a stream vector ; there 

 must be no accumulation ; the current can have no con- 

 vergence. The flow through the aperture means the flow 

 across some surface which abuts on the aperture, and must 

 be the same for all such surfaces, otherwise the expression 

 has no meaning. This states that all currents are circuital 

 in the sense that if we take a tube of flow it is always full, 

 and either comes back on itself or goes to infinity. If we 

 define a current as a flow of electricity this is by no means 

 always true. For example, in the discharge of a condenser 

 with a vacuum dielectric we should have an open current. 

 Maxwell's hypothesis, made to simplify the theory, was that 

 even in this case some sort of release of strain in the aether 

 takes place, which has the electrodynamic properties of a 

 movement of electricity. It is not a movement of elec- 

 tricity ! 



It is shown in the theory of this subject that this fictitious 

 current must be measured by the time rate of change of the 

 electric force. 



We have thus got to make a distinction between the true 

 current, which is a true flow of electricity (or electrons) and 

 this fictitious current in the aether. The contrast is between 

 total and true current. The total current contains a part 

 which is not true electric flow at all, but is a something 

 possessing the electrodynamic properties of a true electric 

 flow. 



2. Faraday's Circuital Relation. 

 The line integral of electric force round a circuit is 

 l—J multiplied into the rate of decrease of the total induction 



through its aperture. 



The relation implies that the tubes of magnetic induction 

 are circuital: the total induction is the same whatever surface 

 it is measured on. In the case of iron, nickel, or any mag- 

 netizable body, this brings in a distinction. If a magnetic 

 metal were present the ordinary magnetic force is, as we 



