OF ENERGY IN THE ELECTROMAGNETIC FIELD. 
357 
that a metal B C of the iron type, and with temperature falling from B to C, forms 
part of a circuit between two neutral metals of the lead type A B and C D, fig. 6, and 
let us further, for simplicity, suppose that these metals are each at the neutral 
temperatures with respect to B C, so that there is no E.M.I. at the junction. If we 
drive a current from A to D by means of some external E.M.I., say at a junction else¬ 
where in the circuit, the potential will tend to fall from A to D. But a current in 
iron from hot to cold cools the metal, that is, the E.M.I. appears to be in opposition 
to the current, so that the energy moves outwards. The potential, therefore, tends to 
rise from B to C, and actually will do so if the resistance of B C is negligable com¬ 
pared with that of the rest of the circuit. In this case the level surfaces will probably 
be somewhat as indicated in the figure (6), where they are numbered in order, each 
surface which cuts B C also cutting A B and C D, and the energy moving outwards 
will come into the circuit again at the parts of A B and C D near the junctions, where 
it will be transformed once more into heat. If the resistance of B C be gradually 
increased the fall of potential, according to Ohm’s law, will tend to lessen the rise, and 
fewer surfaces will cut B C. It would seem possible so to adjust matters that the two 
exactly neutralised each other so that no energy either entered or left B C. In this 
case we should only have lines of magnetic force round B C, and no other characteristic 
of a current in that part of the circuit. 
If this is the true account of the Thomson effect it would appear that it should be 
described not as an absorption of heat or development of heat by the current but 
rather as a movement of energy outwards or inwards, according as the E.M.I. in the 
unequally heated metal opposes or agrees with the direction of the current. 
(5.) A circuit containing a motor. 
This case closely resembles the third case of a circuit containing a copper-zinc cell, 
the motor playing a part analogous to that of the surface of contact of the acid with 
the copper. Let us, for simplicity, suppose that the motor has no internal resistance. 
When it has no velocity all the level surfaces cut the circuit, and the energy leaving 
the dynamo or battery is all transformed into heat due to resistance. But if the 
motor is being worked the current diminishes, the level surfaces begin to converge on 
the motor and fewer cut the circuit. Some of the energy therefore passes into the 
motor, and is there transformed into work. As the velocity increases the number 
cutting the rest of the circuit decreases, for the current diminishes, and, therefore, by 
Ohm’s law, the fall of potential along the circuit is less; and ultimately when the 
* Perhaps this is only true of the wire as a whole. If we could study the effects in minute portions 
it is possible that we should find the seat of the E.M.I. due to difference of temperature not the same as 
that which neutralises it, which is according to Ohm’s law. One, for instance, might be between the 
molecules, the other in their interior, so that there might be an interchange of energy still going on, 
though no balance remained over to pass out of the wire. 
