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PROFESSOR J. H. POYNTING ON THE TRANSFER 
where it h perpendicular to the level surfaces. Now connect A. and B by a fine wire 
L M N of very great resistance, following a line of force and with the resistance so 
adjusted that it is the same for the same fall of potential throughout. We have 
supposed this arrangement of the resistance so that the level surfaces shall not be 
disturbed by the flow of the current. The wire is to be supposed so fine that the 
discharge takes place very slowly. 
While the discharge goes on a current flows round L M N in the direction indicated 
by the arrow, and there is also an equal displacement current from B to A due to the 
yielding of the displacement there. The current will be encircled by lines of magnetic 
force, which will in general form closed curves embracing the circuit. The direction 
of these round the wire will be from right to left in front, and round the space between 
A and B from left to right in front. The E.M.T. is always from the higher level 
surfaces—those nearer A, to the lower—those nearer B, both near the wire and in the 
space between A and B. 
Now, since the energy always moves perpendicularly to the lines of E.M.I. it must 
travel along the equipotential surfaces. Since it also moves perpendicularly to the 
lines of M.I. it moves, as we have seen in (1), inwards on all sides to the wire, and is 
there all converted into heat—if we suppose the discharge so slow that the current is 
steady during the time considered. But between A and B the E.M.I. is opposed to 
the current, being downwards, while the M.I. bears the same relation to the current 
as in the wire. Bemembering that E.M.I., M.I., and direction of flow of energy are 
connected by the right-handed screw relation, we see that the energy moves outwards 
from the space between A and B. As then the strain of the dielectric between A and 
B is gradually released by what we call a discharge current along the wire L hi N, 
the energy thus given up travels outwards through the dielectric following always the 
equipotential surfaces, and gradually converges once more on the circuit where the 
surfaces are cut by the wire. There the energy is transformed into heat. It is to be 
noticed that if the current may be considered steady the energy moves along at the 
same level throughout. 
(3.) A circuit containing a voltaic cell. 
When a circuit contains a voltaic cell we do not know with certainty what is the 
distribution of potential, but most probably it is somewhat as follows A—Suppose we 
* It seems probable that the only legitimate mode of measuring the difference of potential between 
two points in a circuit consisting of dissimilar conductors carrying a steady current, consists in finding 
the total quantity of energy given out in the part of the circuit between the two points while unit 
quantity of electricity passes either point. If this is the case, it seems impossible that the surface of 
contact of dissimilar metals can be the chief seat of the electromotive force, for we have only the very 
slight evolution or absorption of energy there due to the Peltier effect. I have therefore adopted the 
theory of the voltaic circuit in which the seat of at least the chief part of the electromotive force is at 
the contact of the acid and metals. The large differences of potential found by electrometer methods 
