Thomson's Theory

The theory of moving tubes of force has been extensively developed by Sir Joseph Thomson.[38]

Faraday introduced 2 kinds of tubes:

1. Magnetic
2. Electric

These were also used by Poynting.

Thomson resolved to discard the magentic and employ only the electric. This was a distinct departure from Faraday’s conceptions which put great significance to magnetic lines.

Thomson justified his choice by inferences drawn from the phenomena of electric conduction in liquids and gases.

These phenomena indicate that molecular structure is closely connected with tubes of electrostatic force. It was perhaps much more closely than with tubes of magnetic force.

He therefore decided to:

• regard magnetism as the secondary effect
• ascribe magnetic fields to the motion of electric tubes, not to the presence of magnetic tubes.

He assumed that magnetic fields without any electric force came from many tubes that exist everywhere in space, either:

• in the form of closed circuits or
• terminating on atoms

He assumed that electric force is only perceived when the tubes have a greater tendency to lie in one direction than in another.

In a steady magnetic field, the positive and negative tubes might be conceived to be moving in opposite directions with equal velocities.

A beam of light is simply as a group of tubes of force which are moving at the speed of light at right angles to their own length.

His theory is a return to the corpuscular theory.

The tubes have definite directions perpendicular to the direction of propagation. This would easily explain polarization.

Thomson supposed that the energy accompanying all electric and magnetic phenomena was ultimately the kinetic energy of the aether.

1. The electric part of it is represented by rotation of the aether inside and around the tubes.
2. The magnetic part was the energy of the additional disturbance set up in the aether by the movement of the tubes.

• The inertia of this latter motion he regarded as the cause of induced electromotive force.

But this theory could not explain the ponderomotive force which is exerted by the field on a conductor carrying an electric current.

Any ponderomotive force consists in a transfer of mechanical momentum from the agent which exerts the force to the body which experiences it.

Thomson thought that the ponderomotive forces of the electromagnetic field might be explained if the moving tubes of force, which enter a conductor carrying a clurent and are there dissolved, were supposed to possess mechanical momentum, which could be yielded up to the conductor.

Such momentum must be directed at right angles to the tube and to the magnetic induction—a result which suggests that the momentum stored in unit volume of the aether may be proportional to the vector-product of the electric and magnetic vectors.

For this conjecture reasons of a more definite kind may be given.[39]

The ponderomotive forces [40] on material bodies in the electromagnetic field may be accounted for by Maxwell’s supposition that across any plane in the aether whose unit normal is N, there is a stress represented by

…

So long as the field is steady (i.e. electrostatic or magnetostatic) the resultant of the stresses acting on any element of volume of the aether is zero, so that the element is in equilibrium. But when the field is variable, this is no longer the case. The resultant stress on the aether contained within a surface S is

…

integrated over the surface: transforming this into a volume- integral, the term (D.N)E gives a term div D.E + (D.∇)E, where ∇ denotes the vector operator (∂/∂x, ∂/∂y, ∂/∂z); and the first of these terms vanishes, since D is a circuital vector; the term -

(D.E)N gives in the volume-integral a term

…

grad (D.E);

The magnetic terms give similar results. So the resultant force on unit-volume of the aether is

…

which may be written

…

or, by virtue of the fundamental equations for dielectrics,

…

This result compels us to adopt one of 3 alternatives:

1. Modify the theory so as to reduce to zero the resultant force on an element of free aether

This was not favored. [41]

2. Assume that the force in question sets the aether in motion.

This alternative was chosen by Helmholtz[42]. But it is inconsistent with the theory of the aether which was generally received in the closing years of the century.

3. [43] Accept that the aether is itself the vehicle of mechanical momentum, of amount [D.B] per unit volume.

Thomson preferred this.

Maxwell’s theory was now being developed in ways which could scarcely have been anticipated by its author.

But although every year added something to the superstructure, the foundations remained much as Maxwell had laid them, the doubtful argument by which he had sought to justify the introduction of displacement-currents was still all that was offered in their defence.