12 



NATURE 



[May 7, 1891 



HERTZ'S EXPERIMENTS?- 



II. 



T N the last article, a general method of measuring the 

 -*■ velocity at which a disturbance is propagated was 

 described. It depended on being able to produce a 

 regular succession of disturbances at equal intervals of 

 time. These were made to measure their own velocity 

 by reflecting them at an obstacle. Then, by the inter- 

 ference of the incident and reflected waves, a succession 

 of loops and nodes are produced at intervals of half the 

 distance a disturbance is propagated during the time 

 between two disturbances. It is a general method ap- 

 plicable to any sort of disturbance that takes time to get 

 from one place to another. It has been applied over and 

 oyer again to measure the rate at which various kinds of 

 disturbance are propagated in solids, liquids, and gases. 

 It was applied in a modified form years ago, to measure 

 the length of a wave of light ; and, within the last year, 

 some of the most beautiful experiments on photography 

 ever described are applications of this principle by Herr 

 Wiener and M. Lippmann. 



There are three things essential to this experiment : 

 (l) some method of originating waves ; (2) some method 

 of reflecting them ; (3) some method of telling where 

 there are loops and where there are nodes. We will take 

 them in this order: — 



(i) How can we expect to originate electric waves ? If, 

 when a body is electrified positively, the electric force due 

 to it exists simultaneously everywhere, of course we cannot 

 expect to produce anything like a wave of electric force tra- 

 velling out from the body ; but if, when a body is suddenly 

 electrified, the electric force takes time to reach a place, 

 we must suppose that it is propagated in some way as a 

 wave of electric force from the body to the distant place. 

 This, of course, assumes that there is a medium which is 

 in some peculiar state when electric force exists in it, and 

 that it is this peculiar state of the medium, which we call 

 electric force, existing in it, that is propagated from one 

 place to another. It must be carefully borne in mind 

 what sort of a thing this is that we call the electric force 

 at any place. It is not a good name— electric intensity 

 would be a better one ; but electric force has come so 

 much into use, it is hardly to be expected that it can 

 be eradicated now. Electric force at any place is mea- 

 sured by the mechanical force that would be exerted at 

 the place if a unit quantity of electricity were there. It 

 IS not a force itself at all ; it is only a description of the 

 condition of the medium at the place which makes elec- 

 tricity there tend to move. I'he air near the earth 

 IS in such a condition that everything immersed in it 

 tends to move away from the earth with a force of about 

 I '26 dynes for each cubic centimetre of the body, i.e. 

 each cubic centimetre tends to move with a fo:ce of 1-26 

 dynes. Now the condition of the air that causes this 

 is never described as volume force existing at the place, 

 though we do describe the corn sponding condition of the 

 ether as electric force existing there : and as volume force 

 existing would be a very objectionable description of the 

 condition of the air when, being at different pressures at 

 various levels, it tends to make bodies move with a force 

 proportional to their volume, so electric force existing is 

 a very objectionable description of the condition of the 

 ether, whatever it is, that tends to make bodies move 

 with a force in proportion to their electric charges. We 

 know more about the structure of the air than we do 

 about the ether. We know that the structure of the air 

 that causes it to act in this way is that there are more 

 rnolecules jumping about in each cubic centimetre near 

 the earth than there are at a distance, and we do not 

 know yet what the structure of the ether is that causes it 

 to act in this remarkable way ; but even though we do 



' Continued from vol. x'aii. p. 538. 

 NO. I 123, VOL. 44] 



not know the nature of the structure, we know some of 

 its effects, by means of which we can measure it, and we 

 can give it a name. Although we know very little indeed 

 about the structure of a piece of stressed india-rubber, 

 yet we can measure the amount of its stress at any 

 place, and can call the india-rubber in this peculiar 

 condition "stressed india-rubber." As a matter of 

 fact, we know a great deal more about the peculiar con- 

 dition of the ether that we describe as "electric force" 

 existing than we do about the "stressed india-rubber"; 

 and there is every reason to suppose that the structure of 

 the ether is, out of all comparison, more simple than that 

 of india-rubber. 



When sound-waves travel through the air, they consist 

 of compressions followed by rarefactions, and between 

 them the pressure varies from point to point, so that here 

 we have travelling forward a structure the same as that 

 of the air near the earth, and waves of sound might be 

 described as consisting of a succession of positive and 

 negative " volume forces " travelling forward in the air : 

 this form of expression would no doubt be objectionable, 

 but still if all we knew about the properties of the air 

 near the earth was that it tended to make bodies move 

 away from the earth with a force proportional to their 

 volume, it is quite likely that this condition of affairs 

 near the earth might have been described as the existence 

 of a " volume force " near the earth, and when it was dis- 

 covered that this action was due to a medium, the air, it 

 would have been quite natural to describe this state of 

 the air as " volume force " existing in it : and then when 

 waves of sound were observed it would be quite natural 

 that they should be described as waves of " volume force," 

 especially if the only way in which we could detect the 

 presence of these waves was by observing the force 

 exerted on bodies immersed in it, which was proportional 

 to their volumes, and which we happen to know is really 

 due to differences of pressure at neighbouring points in 

 the air. We do not know what is the structure of the 

 ether that causes it to exert force on electrified bodies, 

 but we know of the existence of this property, and when 

 it is in this state we say that "electric force" exists in it, 

 and we have certain ways by which we can detect the 

 existence of "electric force," one of which is the produc- 

 tion of an electric current in a conductor, and the con- 

 sequent electrification of the conductor, and if this is 

 strong enough we can produce an electric spark between 

 it and a neighbouring conductor. When a conductor is 

 suddenly electrified, the structure of the ether which is 

 described as electric force existing in it travels from its 

 neighbourhood through the surrounding ether, and this 

 is described as a wave of electric force travelling through 

 the surrounding ether. It is desirable to be quite clear 

 as to what is meant by the term a wave of electric force, 

 and what we know about it. We know that it is a region 

 of ether where its structure is the same as in the neigh- 

 bourhood of electrified and some other bodies, and owing 

 to which force is exerted on electrified bodies, and electric 

 currents are produced in conductors. 



We may, then, reasonably expect that, if it is possible 

 to electrify a body alternately positively and negatively 

 in rapid succession, there will be produced all round it 

 waves of electric force— that is, if the electric force is 

 propagated by, and is due to, a medium surrounding the 

 electrified body, if electrification is a special state of the 

 medium that fills the space between bodies. 



(2) The next question is : How can we reflect these 

 waves .? In order to reflect a wave, we must interpose in 

 its way some body that stops it. What sort of bodies 

 stop electric force.? Conductors are known to act as 

 complete screens of electric force, so that a large con- 

 ducting sheet would naturally be suggested as the best 

 way to reflect waves of electric force. Reflection always 

 occurs when there is a change in the nature of the 

 medium, even though the change is not so great as to 



