electric induction through a curve is caused by the passage of tubes of force in or out across the boundary; so that whenever magnetomotive force is produced by change in the electric field, or by motion of matter through the field, the magnetomotive force is proportional to the number of tubes of electric force intersected by unit length in unit time.
Poynting, moreover, assumed that when a steady current C flows in a straight wire, C tubes of electric force close in upon the wire in unit time, and are there dissolved, their energy appearing as heat. If E denote the magnitude of the electric force, the energy of each tube per unit length is 12E, so the amount of energy brought to the wire is 12CE per unit length per unit time. This is, however, only half the energy actually transformed into heat in the wire: so Poynting further assumed that E tubes of magnetic force also move in per unit length per unit time, and finally disappear by contraction to infinitely small rings. This motion accounts for the existence of the electric field; and since each tube (which is a closed ring) contains energy of amount 12C, the disappearance of the tubes accounts for the remaining 12CE units of energy dissipated in the wire.
The theory of moving tubes of force has been extensively developed by Sir Joseph Thomson.[1] Of the two kinds of tubes—magnetic and electric—which had been introduced by Faraday and used by Poynting, Thomson resolved to discard the former and employ only the latter. This was a distinct departure from Faraday's conceptions, in which, as we have seen, great significance was attached to the physical reality of the magnetic lines; but Thomson justified his choice by inferences drawn from the phenomena of electric conduction in liquids and gases. As will appear subsequently, these phenomena indicate that molecular structure is closely connected with tubes of electrostatic force—perhaps much more closely than with tubes of magnetic force; and Thomson therefore decided to regard
- ↑ Phil. Mag. xxxi (1891), p. 149; Thomson's Recent Researches in Elect, and Mag. (1893), chapter i.
