Nov, 3, 1887] 



NATURE 



both — not only at the surface of the wires like electrostatic 

 effects, but all through their substance. This is proved 

 by the fact that conductivity increases in simple proportion 

 with sectional area ; it is also proved by every part of a 

 conductor getting hot ; and it is further proved in the case 

 of liquids by their decomposition. 



But the equally manifest facts of current attraction and 

 current induction prove that the effect of the current is 

 felt throughout the surrounding medium as well, and that 

 its intensity depends on the nature of that medium ; we 

 are thus wholly prevented from ascribing the phenomenon 

 of self-induction or extra-current to simple and straight- 

 forward inertia of electricity in a wire like that of water 

 in a pipe. 



We are thus brought face to face with another sugges- 

 tion to account for these effects, viz. this : Since the 

 molecules of a dielectric are inseparably connected with 

 electricity, and move with it, it is possible that electricity 

 itself has no inertia at all, but that the inertia of the atoms 

 of the displaced dielectric confer upon it the appear- 

 ance of inertia. Certainly they do sometimes confer 

 upon it this appearance, as we see in the oscillatory 

 discharge of a Leyden jar. For a displaced thing to over- 

 shoot its mean position and oscillate till it has expended 

 all its energy, is a proceeding eminently characteristic of 

 inertia ; and so, perhaps, the phenomena of self-induction 

 are similarly, though not so simply, explicable. 



Further consideration of this difficult part of the 

 subject is however best postponed to Part III. 



Energy of the Current. 



I have now called attention to the fact that the whole 

 region surrounding a circuit is a field of force in which 

 many of the most important properties of the current 

 (the magnetic, to wit) manifest themselves. But directly 

 we begin thus to attend to the whole space, and not only 

 to the wires and battery, a very curious question arises. 

 Are we to regard the current in a conductor as propelled 

 by some sort of end-thrust, like water or air driven through 

 a pipe by a piston or a fan, or are v/e to think of it as 

 propelled by side forces, a sort of lateral drag, like water 

 driven along a trough by a blast of air or by the vanes ot 

 paddle-wheels dipping into \\.} Or, again, referring to 

 the cord models. Figs. 5, 6, and 13, were we right in pictur- 

 ing the driving force of the battery as located and applied 

 where shown in the diagrams, or ought we to have schemed 

 some method for communicating the power of the battery 

 by means of belts or other mechanism to a great number 

 of points of the circuit.? 



Prof. Poynting has shown that, on the principles 

 developed by Maxwell, the latter of these alternatives, 

 though apparently the more complicated, is the true one ; 

 and he has calculated the actual paths by which the energy 

 is transmitted from the battery to the various points of a 

 circuit, for certain cases. 



We must learn, then, to distinguish between the flow 

 of electricity and the flow of electric energy : they do not 

 occur along the same paths. Hydraulic analogies, at 

 least hydraulic analogies of a simple kind, break down 

 here. When hydraulic power or steam power is conveyed 

 along pipes, the fluid and its energy travel together. 

 Work is done at one end of the tube in forcing in more 

 water, and this is propagated along the tube and reappears 

 at the distant end as the work of the piston. But in 

 electricity it is not so. Electric energy is not to be re- 

 garded as pumped in at one end of a conducting wire, and 

 as exuding in equal quantities at the other. The electricity 

 does indeed travel thus — whatever the travel of electricity 

 may ultimately be found to mean — but the energy does 

 not. The battery emits its energy, not to the wire direct, 

 but to the surrounding medium ; this is disturbed and 

 strained, and propagates the strain on from point to point 

 till it reaches the wire and is dissipated. This, Prof 



Poynting would say, is the function of the wire : it is to 

 dissipate the energy crowding into it from the medium, 

 which else would take up a static state of strain and 

 cease to transmit any more. It is by the continuous 

 dissipation of the medium's energy into heat that con- 

 tinuous propagation is rendered possible. 



The energy of a dynamo does not therefore travel to a 

 distant motor through the wires, but through the air. The 

 energy of an Atlantic cable battery does not travel to 

 America through the wire strands, but through the 

 insulating sheath. This is a singular and apparently 

 paradoxical view, yet it appears to be well founded. 



Think of a tram-car drawn by an underground rope, 

 like those in the streets of Chicago or Hampstead Hill, 

 A contact piece of iron protrudes from the bottom of the 

 car and grips the moving rope, which is thus enabled to 

 propel the car. How does the energy of the distant 

 stationary engine reach the car t Via the rope and the iron 

 connector, undoubtedly. They both have to be strong, 

 and are liable to be broken by the transmitted stress. 



Next, think of an electric tram-car driven by means of a 

 current taken up from an underground conductor, like 

 that of Mr. Holroyd Smith at Manchester, or at the late 

 Inventions Exhibition. A contact piece of wire rope 

 protrudes from the bottom of the car and drags a little 

 truck along the conductor, which is thus enabled to 

 supply electricity to the electro-magnetic motor geared 

 to the wheels. How does the energy of the distant 

 dynamo reach the car in this case ? Not via the wire 

 connector ; not even via the underground conductor. It 

 travels from the distant dynamo through the general 

 insulating medium between cable and earth, some little 

 enters the conductor and is dissipated, but the great bulk 

 flows on and converges upon the motor in the car, which 

 is thus propelled. All the energy of the conducting wire 

 is dissipated and lost as heat : it is the energy of the 

 insulating medium which is really transmitted and 

 utilized. 



Pheno77iena peculiar to a Starting, or Stopping, or 

 Varying Current. 



There is a remarkable fact concerning electric currents 

 of varying strength, which has been lately brought into 

 prominence by the experimental skill of Prof. Hughes, 

 viz. that a current does not start or stop equally and 

 simultaneously at all points in the section of a conductor, 

 but starts at the outside first. This fact is naturally more 

 noticeable with thick wires than with thin, and it is 

 especially marked in iron wires, for reasons which in 

 Part III. will become apparent ; but the general cause of 

 it in ordinary copper wires can very easily be perceived 

 in the light of the views of Prof. Poynting just mentioned. 

 For, remember that a current in a wire is not pushed 

 along by a force applied at its end, so as to be driven 

 over obstacles by its own momentum combined with a 

 vis a tergo ; but it is urged along at every point of its 

 course by a force just sufficient to make it overcome the 

 resistance there, and no more, the force being applied 

 to it through the medium of the dielectric in which the 

 wire is immersed. A lateral force it is which propels the 

 electricity ; and it naturally acts first on the outer layers 

 of the wire or rod, only acting on the interior portions 

 through the medium of the outside. 



To illustrate this matter further, rotate a common 

 tumbler of liquid steadily for some time and watch the 

 liquid ; dusting powder perhaps over it to make it more 

 visible. You will see first the outer layer begin to particip- 

 ate in the motion, and then the next, and then the next, 

 and so on, until at length the whole is in rotation. Stop 

 the tumbler, and the liquid also begins gradually to stop by 

 a converse process. 



If the liquid sticks together pretty well, like treacle, 

 the motion spreads very rapidly : this corresponds to a 



