ELECTRIC AND MAGNETIC INDUCTIONS IN THE SURROUNDING EIELD. 279 
instance, suppose a small electrified body placed in a field where there is electric 
intensity ; then the body will be acted on by force and will receive energy which 
appears as the energy of motion, the electric energy at the same time decreasing. If 
energy has position that which is now in the body must have come into it through 
the surrounding space, or it was present in that space before the body took it up. 
The alternative that it appeared in the body without passing through the space 
immediately surrounding the body need not be discussed. Hence the existence of 
electric intensity implies the existence of electric energy in the place where the 
electric intensity is capable of manifestation. Similarly magnetic energy accompanies 
magnetic intensity. The inductive condition of the medium imagined by Faraday is 
due then to its modification when containing energy. Maxwell has shown that all 
the energy is accounted for on the supposition that the electric energy per unit 
volume at any point is K(E.I.) 3 /87r, and that the magnetic energy is p(M.I.) 3 /87r. He 
has given in his ‘ Elementary Treatise on Electricity/ p. 47, another way of describing 
the distribution of energy which will be more useful for my purpose. If the field be 
mapped out by unit induction tubes—either electric or magnetic— i.e., tubes drawn 
so that the total induction over every cross section of a tube is unity, and if these 
tubes be divided into cells of length such that the difference of potential or the line 
integral of the intensity between the two ends of each cell is unity, then each cell 
contains, if electric, half a unit of energy, if magnetic of a unit, the divisor 4-7T 
being introduced by the difference in definition of the two inductions. Maxwell 
terms these unit cells. 
II. The second principle is in part experimental, viz.:—that the line integral of 
the electric intensity round any closed curve is equal to the rate of decrease of the 
total magnetic induction through the curve. This is verified by experiment when the 
curve is drawn through conducting material. Maxwell supposes it to be true in all 
cases, that is, he supposes that electric induction can be produced in insulators by 
means of magnetic changes, without the presence of charges on conductors, and is 
therefore led to identify the growth and decrease of electric induction with current. 
III. The third principle is also in part experimental, viz.:—that the line integral of 
the magnetic intensity round any closed curve is equal to 47 tX current through the 
curve. This is verified by experiment when the current is in a wire, and Maxwell 
supposes it to be also true in the case where there is change of electric induction in an 
insulator. The supposition is justified by Prof. Howland’s well-known experiment. 
From these three principles Maxwell deduces his general equations of the Electro¬ 
magnetic Field. I have stated them in full as I propose to modify the second and 
third principles, and I wish to make quite clear the nature of the proposed changes. 
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