392 



NATURE 



\August 23, 1888 



speed directly, but we can try to observe it experi- 

 mentally. 



The first and crudest way of making the attempt would 

 be to arrange a secondary circuit near our oscillating 

 primary circuit, and see how soon the disturbance reached 

 it. For instance, we might take a nearly closed loop, 

 make it face a Leyden jar circuit across a measured dis- 

 tance, and then look for any interval of time between the 

 spark of the primary discharge and the induced spark of 

 the secondary circuit, using a revolving mirror or what 

 we please. But in this way we should hardly be able to 

 detect any time at all : the propagation is too quick. 



We might next make use of the principle of the electric 

 telegraph, viz. the propagation of a disturbance round a 

 single circuit from any one point of origin. Consider a 

 large closed circuit, either conveying or not conveying a 

 current : introduce at any one point a sudden change — 

 a sudden E.M.F., for instance, or a sudden resistance if 

 there be a current already. Out from that point a dis- 

 turbance will spread into the ether, just as happens in air 

 when a blow is struck or gun-cotton fired. A regular suc- 

 cession of disturbances would carve the ether into waves : 

 a single disturbance will merely cause a pulse or shock ; 

 but'the rate of transmission is the same in either case, 

 and we may watch for the reception of the pulse at a 

 distant station. If the station has to be very distant in 

 order to give an appreciable lapse of time, a speaking- 

 tube is desirable to prevent spreading out in all directions 

 — to concentrate the disturbance at the desired spot. 

 What a speaking-tube is to sound, that is the wire of the 

 circuit — the telegraph wire — to ethereal pulses. 



It is a curious function, this of the telegraph wire : it 

 does not convey the pulses, it directs them. They are 

 conveyed wholly by the ether, at a pace determined by 

 the properties of the ether, modified as it may be by the 

 neighbourhood of gross matter. Any disturbance which 

 enters the wires is rapidly dissipated into heat, and gets 

 no further ; it is the insulating medium round it which 

 transmits the pulses to the distant station. 



All this was mentioned in Part III., and an attempt 

 was made to explain the mechanism of the process, and to 

 illustrate in an analogical way what is going on. 



The point of the matter is that currents are not propelled 

 by end-thrusts, like water in a pipe or air in a speaking- 

 tube, but by lateral propulsion, as by a series of rotating 

 wheels with their axes all at right angles to the wire sur- 

 rounding it as a central core, and slipping with more or 

 less friction at its surface. This is characteristic of ether 

 modes in general : it does not convey longitudinal waves 

 or end-thrust pulses, like sound, but it conveys transverse 

 vibrations or lateral pulses, like light. 



Without recapitulating further, we can perceive, then, 

 that the transmission of the pulse round the circuit to its 

 most distant parts depends mainly on the medium sur- 

 rounding it. The process is somewhat as follows : — Con- 

 sider two long straight parallel wires, freely suspended, 

 and at some great distance joined together. At the near 

 end of each, start equal opposite electromotive impulses, 

 as by suddenly applying to them the poles of a battery ; 

 or apply a succession of such pulses by means of an 

 alternating machine. Out spread the pulses into space, 

 starting in opposite phases from the two wires, so that at 

 a distance from the wires the opposite pulses interfere with 

 each other, and are practically non-existent, just as but 

 little sound is audible at a distance from the two prongs 

 of a freely suspended tuning-fork. But near the wires, 

 and especially between them, the disturbance may be 

 considerable. To each wire it spreads and is dissipated, 

 and so a fresh supply of energy goes on continually 

 arriving at the wires, always flowing in from outside, 

 to make up the deficiency. If the wires are long enough 

 hardly any energy may remain by the time their distant 

 ends are reached ; but whatever there is will still be crowd- 

 ing in upon the wires and getting dissipated, unless by 



some mechanism it be diverted and utilized to effect some 

 visible or audible or chemical change, and so to give the 

 desired signal. 



Now the pace at which this transmission of energy 

 goes on in the direction of the wires is pretty much 

 the same as in free space. There are various circum- 

 stances which can retard it ; there are none which can 

 accelerate it. The circumstances which can retard it 

 are, first, constriction of the medium by too great proximity 

 of the two conducting wires : as, for instance, if they con- 

 sisted of two flat ribbons close together with a mere film of 

 dielectric between, or if one were a small-bore tube and 

 the other its central axis or core. In such cases as this 

 the general body of ether takes no part in the process, the 

 energy has all to be transmitted by the constricted portion 

 of dielectric, and the free propagation of ethereal pulses 

 is interfered with : the propagation is no longer a true 

 wave-propagation at all, but approximates more or less 

 closely to a mere diffusion creep, rapid it may be, and yet 

 without definite velocity, like the conduction of heat or the 

 diffusion of a salt into water. One well-known effect of 

 this is to merge successive disturbances into one another, 

 so that their individuality, and consequently the distinctness 

 of signalling, is lost. 



Another circumstance which can modify rate of trans- 

 mission of the pulses is ethereal inertia in the substance 

 of the conducting wires, especally extra great inertia, as, 

 for instance, if they are made of iron. For the dissipa- 

 tion of energy does not go on accurately at their outer 

 surface ; it has usually to penetrate to a certain depth, 

 and until it is dissipated the fresh influx of energy from 

 behind does not fully occur. Now, so long as the value 

 of /x for the substance of the wires is the same as that of 

 air or free space, no important retardation is thus caused, 

 unless the wires are very thick ; but directly the inertia in 

 the substance of the wires is one or two hundred times as 

 big as that outside, it stands to reason that more time is 

 required to get up the needful magnetic spin in its outer 

 layers, and so the propagation of pulses is more or less 

 retarded. At the same time this circumstance does not 

 alter the character of the propagation, it does not change 

 it from true wave velocity to a diffusion, it leaves its 

 character unaltered ; and so the signals, though longer 

 in coming, may arrive quite clear, independent, and dis- 

 tinct. It is much the same, indeed, as if the density of the 

 surrounding medium had been slightly increased. 



These, then, are the main circumstances which affect 

 the rate of transmission of a pulse from one part of a 

 closed circuit to another : extra inertia or so-called mag- 

 netic susceptibility in the conducting substance, especially 

 in its outer layers ; and undue constriction or throttling of 

 the medium through which the disturbance really has to 

 go. Both these circumstances diminish rate of trans- 

 mission, and one (the last mentioned) modifies the law 

 and tends to obliterate individual features and to destroy 

 distinctness. 



Of course, besides these, the nature of the insulating 

 medium will have an effect on the rate of propagation, 

 but that is obvious all along ; it is precisely the rate at 

 which any given medium transmits pulses that we want 

 to know, and on which we are thinking of making experi- 

 ments. If we use gutta-percha (more accurately the ether 

 inside gutta-percha) as our transmitting medium in an 

 experiment, we are not to go and pretend that we have 

 obtained a result for air. 



The circumstances we have considered as modifying the 

 rate of transmission are both of them adventitious circum- 

 stances, independent of the nature of the medium, and 

 they are entirely at our own disposal. If we like to 

 throttle our medium, or to use thick iron wires, we can do 

 so, but there is no compulsion : and if we wish to make 

 the experiment in the simplest manner, we shall do no 

 such thing. We shall use thin copper wires (the thinner 

 the better), arranged parallel to one another a fair distance 



