350 The Followers of Maxwell. 



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 \E, so 

 the amount of energy brought to the wire is \CE 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 J(7, the disappearance of the tubes 

 accounts for the remaining \GE units of energy dissipated in 

 the wire. 



The theory of moving tubes of force has been extensively 

 developed by Sir Joseph Thomson.* 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 electro- 

 static 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. 



