26 



KNOWLEDGE. 



[Februaby 1, 1900. 



the sound, but it is found that the transverse vibrations 

 die away almost immediately. Clerk Maxwell points 

 out that, within a single wave length, the amplitude 

 of the transverse vibrations will be reduced to less than 

 one-five-hundredth of its initial value owing to this 

 state of irregular agitation. 



In the case of light, on the other hand, it is found 

 that transverse vibrations are the only ones which are 

 transmitted, that is to say, that all vibrations along 

 the line in which the wave is travelling die away almost 

 immediately, so that the vibrations are entirely per- 

 pendicular to the line of transmission. The reason of 

 this had never been explained until Maxwell showed, 

 from electromagnetic theory, that electric waves must 

 have this characteristic. This suggested to him the 

 hypothesis, that light waves were simply electric waves, 

 of such wave lengths as to be capable of affecting the 

 receiving instrument commonly known as the eye. 



Many phenomena when investigated were found to 

 confirm this hypothesis ; the close correspondence, for 

 example, between the calculated speed of transmission 

 of an electromagnetic wave and the observed velocity 

 of light; and, again, the fact that transparent sub- 

 stances are invariably bad conductors of electricity. 

 Hertz, however, made a step further, for, as we shall 

 see, he succeeded in producing waves known to be of 

 electro-magnetic origin, and in showing that they could 

 be made to produce interference phenomena and 

 undergo reflection and refraction exactly like light 

 waves. 



When oscillations are set up in an electric circuit it 

 can be shown that the time, T, of a complete oscillation 

 is de'^ermined by the equation 



T = 2 TT y L, S, 



where L and S are two of the electric constants of the 

 circuit known as its self induction and its capacity 

 respectively, while of course * stands as usual for the 

 ratio of the circumference of a circle to its diameter, 

 which is approximately equal to 22/7. 



The speed with which the waves travel, depends only 

 on the medium being equal to the square root of the 

 ratio of its elasticity to its density. In the case of the 

 sether this speed is about 186,000 miles a second, the 

 observed speed of light. 



We will now consider the question as to what kind of 

 jether waves are most suitable for the transmission of 

 signals to a distance. 



The conditions to be fulfilled are clearly two in 

 number. Firstly, in order that the waves may not be 

 stopped by intervening obstacles, such as portions of 

 land and water, we require oscillations for which the 

 opacity of different kinds of matter is least, or, in other 

 words, those oscillations for which ordinary terrestrial 

 bodies are most transparent. 



Secondly, in order that the signals may be dis- 

 tinguishable at as groat distances as possible with a 

 moderate expenditure of energy, we require those os- 

 cillations for which the largest possible proportion of 

 the energy supplied from the source, the transmitting 

 instrument may be taken up by the medium. 



We know that ordinary light waves, the wave lengths 

 of which are measured in hundred-thousandths of an 

 inch, fulfil the second condition in the most satisfactory 

 manner, but unfortunately they do not fulfil the first, 

 for the thinnest films of most substances are sufficient 

 to stop them. Still, they were employed for the earliest 

 attempts at wireless telegraphy, which is far more 

 ancient than the system of telegraphing by means of 



wires. In the earliest examples of which we have any 

 record, the requisite setliereal oscillations were excited 

 by means of large bonfires, and the difficulty of fulfilling 

 the second condition was evaded by placing both the 

 transmitting instrument consisting of the bonfire, and 

 the receiving instrument, which was simply the eye 

 of the watchman, on the highest hills available, so 

 that the waves excited had only to encounter the com- 

 paratively transparent atmosphere. The semaphore of 

 a hundred years ago and the heliograph of to-day offer 

 further examples of wireless telegraphy by means of 

 electric oscillations of extremely short wave length. 



All bodies become less opaque to electric bodies as the 

 wave length inci-eases. The reason of this, according 

 to theory, is that the quenching of the waves does not 

 take place immediately on entering any opaque medium, 

 as would be the case if it were a perfect conductor of 

 electricity, but the waves die out after a certain number 

 of vibrations depending on the opacity of the medium. 



It is clear, therefore, that in the case of a medium 

 which will permit of half-a-dozen vibrations before the 

 wave is quenched, a very thin film will suffice to stop 

 light waves which are of the order of a hundred-thou- 

 sandth of an inch in length, while a much thicker 

 stratum would be required to stop the Hertzian waves 

 which may be from a foot to some few yards in length, 

 while no practicable thickness would stop the waves 

 from an alternating dynamo, say with a periodicity of 

 100 vibrations a second, as in this case the wave length 

 would be something like a couple of thousand miles. 



Unfortunately, as the wave length increases, the 

 second condition is less and less perfectly fulfilled. 



The reason for this is extremely interesting. Sir 

 George Gabriel Stokes, so long ago as 1849, showed by 

 mathematical reasoning from observed optical phe- 

 nomena, that when a wave of light is excited from a 

 given source, the radiation is emitted, not from the 

 source itself, but from a point a quarter wave length in 

 advance of it. This very curious phenomenon is com- 

 pletely explained when light waves are admitted to 

 be of electromagnetic origin. 



When an electric disturbance is set up at a certain 

 point, it is always accompanied by a magnetic disturb- 

 ance in a plaae at right angles to it. The electric 

 disturbance occurs a quarter of a period later than the 

 magnetic, but it starts a quarter of a wave length in 

 advance, so that, except within the first quarter wave 

 length, the two travel together, their zero and maximum 

 values always occurring at the same points. 



Within the first quarter wave length, however, the 

 two disturbai.^es sometimes reinforce and sometimes 

 oppose each other, and the result of this, as Professor 

 Poynting has shown, is that, within the first quarter 

 wave length, the energy originally proceeding from 

 the source of the disturbance is sometimes travelling 

 forward and sometimes backward towards the source, so 

 that, although more goes forward than comes back- 

 ward, a large proportion is wasted. 



Beyond the first quarter wave length, however, the 

 two disturbances tend always to cause an outward flow 

 of energy. 



It is, therefore, easily seen that in the case of a wave 

 a hurdred- thousandth of an inch in length, the point 

 from which the radiation begins being only the four- 

 hundred-thousandth of an inch from the source, there 

 will be very little energy returning to the source. 



On the other hand, in the case of a dynamo such as 

 referred to above, with a wave length of some 2,000 

 miles, the emission point would be sorne 500 miles 



