lO 



NATURE 



{^Nov. 3, 1887 



poor conductor. If the liquid be very mobile, the propa- 

 gation of motion inward is slow : this corresponds to a 

 very good conductor. If the liquid were perfectly non- 

 viscous, it would correspond to a perfect conductor, and 

 no motion would ever be communicated to it deeper 

 than its extreme outer skin. 



Think now of a long endless tube full of water, say the 

 hollow circumference of a wheel, and spin it : the liquid 

 is soon set in rotation, especially if the tube be narrow 

 or the liquid viscous ; but it is set in motion by a lateral 

 not an end force, and its outer layers start first. 



Just so is it with a current starting in a metal wire. If 

 the wire be fine, or its substance badly conducting, it all 

 starts nearly together ; but if it be made pretty thick, and 

 of well conducting substance, its outer layers may start 

 appreciably sooner than the interior. And if it were 

 infinitely conducting, no more than the outer skin would 

 ever start at all. 



In actual practice the time taken for all the electricity 

 in an ordinary wire to get into motion is excessively 

 short — something like the thousandth of a second — 

 so that the only way to notice .the effect is to start and 

 reverse the current many times in succession. 



If the hollow-rimmed wheel above spoken of were made 

 to oscillate rapidly, it is easy to see that only the outer 

 layers of water in it would be moved to and fro ; the inner- 

 most water would remain stationary ; and accordingly 

 it would appear as if the tube contained much less water 

 than it really does. The virtual bore of the pipe would, 

 in fact, for many purposes be diminished. So is it 

 also with electricity ; the sectional area of a wire to a 

 rapidly alternating current is virtually lessened so far as 

 its conducting power is concerned ; and accordingly its 

 apparent resistance is slightly higher for alternating than 

 for steady currents. The effect is however too small to 

 notice in practice except with thick wires and very rapid 

 alternations. 



By splitting up the conductor into a bundle of insulated 

 wires, thus affording the dielectric access to a considerable 

 surface of conductor, the force is applied much more 

 thoroughly, and so the effect spoken of is greatly lessened. 

 The same thing is achieved by rolling out the conducting- 

 rod into a flat thin bar. Making the conductor hollow 

 instead of solid offers no particular advantage, because 

 no energy travels 'via the hollow space, it still arrives 

 only from the outside ; unless, indeed, the return part of 

 the circuit is taken along the axis of the hollow like a 

 telegraph cable. In this last arrangement all the energy 

 travels via the dielectric between the two conductors, and 

 none travels outside at all. It will be perceived therefore 

 that, as in static electricity, the term " outside " must be 

 used with circumspection : it reilly means that side of a 

 conductor which faces the opposite conductor across a 

 certain thickness of dielectric. 



We learn from all this that, whereas in the case of steady 

 currents the sectional area and material of a conductor 

 are all that need be attended to, the case is different when 

 one has to deal with rapidly alternating currents, such as 

 occur in a telephone, or, again, such as are apt to occur 

 in a Leyden-jar discharge (see Part I., p. 560), or in 

 lightning. 



In all these cases it is well to make the conductor ex- 

 pose considerable surface to the propelling medium — the 

 dielectric — else will great portions of it be useless. 



Hence, a lightning-conductor should not be a round 

 rod, but a flat strip, or a strand of wires, with the strands 

 as well separated as convenient : and though I have not 

 yet mentioned the special effect of iron, I may as well say 

 here that iron is about 90,000 times worse than copper 

 for the purpose of a lightning-conductor in respect of the 

 phenomenon just described, seven times as bad on account 

 of its inferior conducting power, and about twice as good 

 as copper because of its higher melting-point and specific 

 heat. 



The Question of Electrical Momentum agm'n. 



We are now able to return to the important question 

 whether an electric current has any momentum or not, as 

 it would have if it were a flow of material liquid. Re- 

 ferring to Part I. (p. 533), a hint will be found that the laws 

 of flow of a current in conductors — the shape of the 

 stream-lines, in fact — are such as indicate no inertia, or 

 else no friction. Now Ohm's law shows that at any rate 

 friction is not absent from a current flowing through a 

 metal ; hence it would appear at first sight as if inertia 

 must be absent. 



The stream-lines bear upon the question in the follow- 

 ing kind of way. If an obstacle is interposed in the 

 path of a current of water, the motion of the water is 

 unsymmetrical before and behind the obstacle. The 



'/>„„„„j.i,muu,.„„ , 



Fig. 14. — Stream-lines of water flowing through a pipe with an obsiruction 

 in It. 



stream-lines spread out as the water reaches the obstacle, 

 and then curl round it, leaving a space full of eddies in its 

 wake (Fig. 14). 



But if one puts an obstacle in the path of an electric 

 current — say by cutting a slit in a conducting strip of 

 tinfoil — the stream-lines on either side of it are quite 

 symmetrical, thus — 



'...■■y^>^,J-J>,^JUlJ>,j„J>JllJ,.^.UI»» , V ' IM. g J. 



'r^'mfj/mMt-'mmr. 



Fig. 15. — Electrical stream-lines past an obstacle. 



And this is exactly what would be true for water also,, 

 if only it were devoid either of friction or of inertia, or of 

 both. 



Is not this fact conclusive, then ? Does it not prove 

 the absence of momentum in electricity ? 



Plainly the answer must depend on whether there is 

 any other possible mode of accounting for this kind of 

 flow. And there is. 



For suppose that water, instead of being urged by 

 something not located at or near the obstacle — instead of 

 being left to its own impetus to curl round or shoot past 

 as it pleases — suppose it were propelled by a force acting 

 at every point of its journey, a force just able to drive it 

 at any point against the friction existing at that point and 

 no more ; then the flow of water would take place accord- 

 ing to the electrical stream-lines shown in Fig. 15. 



An illustration of such a case is ready to hand. Take 

 a spade-shaped piece of copper wire or sheet, heat it a 

 little, and fix it in quiescent smoky air ; looking along it 

 through a magnifier in a strong light you will see the 

 warmed air streaming past the metal according to the 

 stream-lines of Fig. 15 ; and this just because the moving 

 force has its location at the metal surface, and not in some 

 region below it. (See Lord Rayleigh, NAxaRE, vol. xxviii. 

 p. 139). One cannot indeed say that it is propelled at every 



