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PROFESSOR J. H. POYNTING ON THE TRANSFER 
it is first recognisable as electric and magnetic to the parts where it is changed into 
heat or other forms \ 
The aim of this paper is to prove that there is a general law for the transfer of 
energy, according to which it moves at any point perpendicularly to the plane con¬ 
taining the lines of electric force and magnetic force, and that the amount crossing 
unit of area per second of this plane is equal to the product of the intensities of the 
two forces multiplied by the sine of the angle between them divided by 4 t r, while the 
direction of flow of energy is that in which a right handed screw would move if turned 
round from the positive direction of the electromotive to the positive direction of the 
magnetic intensity. After the investigation of the general law several applications 
will be given to show how the energy moves in the neighbourhood of various current¬ 
bearing circuits. 
The following is a general account of the method by which the law is obtained. 
If we denote the electromotive intensity at a point (that is the force per unit of 
positive electrification which would act upon a small charged body placed at the point) 
by (5, and the specific inductive capacity of the medium at that point by I\, the 
magnetic intensity (that is, the force per unit pole which would act on a small north- 
seeking pole placed at the point) by and the magnetic permeability by fi, 
Maxwell’s expression for the electric and magnetic energies per unit volume of the 
field is 
Kd' 3 /^ 7r 'h/ x '§V^ 7r .(1) 
If any change is going on in the supply or distribution of energy the change in this 
quantity per second will be 
KQt^liTT + ^UTT .(2) 
According to Maxwell the true electric current is in general made up of two parts, 
one the conduction current and the other due to change of electric displacement in 
the dielectric, this Tatter being called the displacement current. Now, the displace¬ 
ment is proportional to the electromotive intensity, and is represented by K(S/ 47 t, so 
that when change of displacement takes place, due to change in the electromotive 
intensity, the rate of change, that is, the displacement current, is Iv /4 tt, and this is 
cl T/ 
equal to the difference between the true current and the conduction current J\. 
Multiplying this difference by the electromotive intensity (§ the first term in ( 2 ) 
becomes 
= .(3) 
47 t at 
The first term of the right side of (3) may be transformed by substituting for the 
components of the total current their values in terms of the components of the 
magnetic intensity, while the second term, the product of the conduction current 
