that a metal BC 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 AB and CD, fig. 6, and let us further, for simplicity, suppose that these metals are each at the neutral temperatures with respect to BC, 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 elsewhere 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 BC is negligible compared 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 BC also cutting AB and CD, and the energy moving outwards will come into the circuit again at the parts of AB and CD near the junctions, where it will be transformed once more into heat. If the resistance of BC be gradually increased the fall of potential, according to Ohm's law, will tend to lessen the rise, and fewer surfaces will cut BC. It would seem possible so to adjust matters that the two exactly neutralised each other so that no energy either entered or left BC. In this case we should only have hues of magnetic force round BC, and no other characteristic of a current in that part of the circuit.[1]
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.
