August 23, 1888] 



NA TURE 



>9i 



standing the exact function and importance of every 

 portion of the surrounding space. 



Corresponding, then, to the well-known simple harmonic 



T = 27r v/-t> we have, writing L for the self-induction 



or inertia of the circuit, and S for its capacity or inverse 

 rigidity constant, 



T = 27r^LS, 



This, therefore, is the time of a complete swing. Directly 

 the jar is discharged, these oscillations begin, and they 

 continue like the vibration of a tuning-fork until they are 

 damped out of existence by viscosity and other modes of 

 dissipation of energy. 



But now just consider a tuning-fork. Suppose its sub- 

 stance were absolutely unviscous, would it go on vibrating 

 for ever? In a vacuum it might : in air it certainly would 

 not. And why not ? Because it is surrounded by a 

 medium capable of taking up vibrations and of propagat- 

 ing them outwards without limit. The existence of a 

 vibrating body in a suitable medium means the carving 

 of that medium into a succession of waves and -the trans- 

 mission of these waves away into space or into absorbing 

 obstacles. It means, therefore, the conveyance away of 

 the energy of the vibrating body, and its subsequent 

 appearance in some other form wherever the radiating 

 waves arc quenched. 



The laws of this kind of wave-propagation are well 

 known ; the rate at which waves travel through the 

 medium depends not at all on any properties of the 

 original vibrating body, the source of the disturbance ; it 

 depends solely on the properties of the medium. They 

 travel at a rate precisely equal to the square root of the 

 ratio of its elasticity to its density. 



Although the speed of travel is thus fixed independently 

 of the source, the length of the individual waves is not so 

 independent. The length of the waves depends both on 

 the rate at which they travel and on the rate at which 

 the source vibrates. It is well known and immediately 

 obvious that the length of each wave is simply equal to 

 the product of the speed of travel into the time of one 

 vibration. 



But not every medium is able to convey every kind of 

 vibration. It may be that the mode of vibration of a 

 body is entirely other than that which the medium 

 surrounding it can convey : in that case no dissipation 

 of energy by wave-propagation can result, no radiation 

 will be excited. The only kind of radiation which 

 common fluids are mechanically able to transmit is well 

 known : it is that which appeals to our ears as sound. 

 The elasticity concerned in such disturbance as this is 

 mere volume elasticity or incompressibility. But electrical 

 experiments (the Cavendish experiment, 1 and Faraday's 

 ice-pail experiment) prove the ether to be enormously 

 — perhaps absolutely — incompressible ; and if so, such 

 vibrations as these would travel with infinite speed and 

 not carve proper waves at all. 



Conceivably (I should like to say probably) gravitation 

 is transmitted by such longitudinal impulses or thrusts, 

 and in that case it is nearly or quite instantaneous ; and 

 the rate at which it travels, if finite, can be determined 

 by a still more accurate repetition of the Cavendish 

 experiment than has yet been made ; but true radiation 

 transmitted by the ether cannot be of this longitudinal 

 character. The elasticity possessed by the ether is of 

 the nature of rigidity : it has to do with shears and distor- 

 tions ; not mechanical stresses, indeed — to them it is quite 

 limpid and resistless — but electromotive stresses : it has an 

 electrical rigidity, and it is this which must be used in the 

 transmission of wave-motion. 



But the oscillatory discharge of a Leyden jar is precisely 

 competent to apply to the ether these electromotive vibra- 

 tions : it will shake it in the mode suitable for it to 



See Maxwell's "Electrical Researches of Cavendish," p. 104 ; see also p. 417. 



transmit ; and accordingly, from a discharging circuil, 

 waves of electrical distortion, or transverse waves, will 

 spread in all directions at a pace depending on the 

 properties of the medium. 



Thus, then, even with a circuit of perfect conductivity 

 the continuance of the discharge would be limited, the 

 energy would be dissipated ; not by friction, indeed — there 

 would in such a circuit be no direct production of heat — 

 it would be dissipated by radiation, dissipated in the same 

 way as a hot body cooling, in the same way as a vibrating 

 tuning-fork mounted on its resonant box. The energy of 

 the vibrating body would be transferred gradually to the 

 medium, and would by this be conveyed out and away, 

 its final destination being a separate question, and 

 depending on the nature and position of the material 

 obstacles it meets with. 



Velocity of Electrical Radiation. 

 The pace at which these radiation-waves travel depends, 

 as we have said, solely on the properties of the medium, 

 solely on the relation between its elasticity and its density. 

 The elasticity considered must be of the kind concerned 

 in the vibrations ; but the vibrations are in this case 

 electrical, and so electrical elasticity is the pertinent kind. 

 This kind of elasticity is the only one the ether possesses 

 of finite value, and its value can be measured by electro- 

 static experiments. Not absolutely, unfortunately : only 

 the relative elasticity of the ether as modified by the 

 proximity of gross substances has yet been measured : 

 its reciprocal being called their specific inductive capacity, 

 or dielectric constant, K. The absolute value of the quan- 

 tity K is at present unknown, and so a convention has 

 arisen whereby in air it is called 1. This convention is 

 the basis of the artificial electrostatic system of ur.ics. 

 No one supposes, or at least no one has a right to sup- 

 pose, that its value is really 1. The only rational guess 



at its value is one by Sir William Thomson, 1 viz. s — -• 



' 842 « 



Whether known or not, the absolute value of the dielectric 

 constant is manifestly a legitimate problem which may 

 any year be solved. 



The other thing on which the speed of radiation waves 

 depends is the medium's density — its electric density, if 

 so it must be distinguished. Here, again, we do not 

 know its absolute value. Its relative or apparent amount 

 inside different substances is measured by magnetic 

 experiments, and called their specific magnetic capacity, 

 or permeability, and is denoted by /*. 



Being unknown, another convention has arisen, quite 

 incompatible with the other convention just mentioned, 

 that its value in air shall be called I. This convention is 

 the basis of the artificial electro-magnetic system of units 

 — volts, ohms, amperes, farads, and the like. Both of 

 these conventions cannot be true : no one has the least 

 right to suppose either true. The only rational guess at 

 ethereal free density is one by Sir William Thomson, viz. 

 9*36 X io' 1;) . 



Very well, then ; it being clearly understood that these 



two great ethereal constants, k or , and /*, are neither of 



K. 



them at present known, but are both of them quite know- 

 able, and may at any time become known, it remains to 

 express the speed of wave transmission in terms of them. 

 But it is well known that this speed is simply the square 

 root of the ratio of elasticity to density, or 



n 1 



This then is the speed with which waves leave the 

 discharging Leyden jar circuit, or any other circuit con- 

 veying alternating or varying currents, and travel out 

 into space. 



Not knowing either, k or /*, we cannot calculate this 



1 Trans. R S. Edin., xxi. Co ; see also article "Ether," in the "Encyc. 

 Brit." 



