558 EVENING DISCOURSES. 
sending station, so that the signals might consist of modifications or modulations in 
its amplitude, which modulations might recur with a frequency of their own after the 
fashion of group waves. To use an acoustic analogy, the method hitherto in vogue 
had been analogous to signalling by strokes on a bell; whereas what was aimed at 
was to get something more like organ notes, which would be continuous except when 
modulated by some kind of keyboard. 
So far as I know, the first method of achieving this was the outcome of an experi- 
ment by Duddell with an electric arc as part of a tuned circuit. The circuit had a 
capacity and self-induction in series with the arc. The are acted very like the blast 
of air on the lip of an organ pipe, giving an irregular flutter or disturbance which 
might be made periodic by a suitable resonant cavity, the resonant cavity or organ 
pipe itself being represented in the electrical case by the self-induction and capacity, 
that is, by the inertia and elasticity of the circuit. So long as the circuit was not 
interfered with, the note emitted was of a uniform tone; but by means of a key the 
self-induction or capacitiy could be varied, and thus the note emitted could be changed. 
In that way with a few keys Duddell was able to carve the continuous wave into an 
imitation of the National Anthem or any other well-known tune. Duddell’s arc, 
however, only responded to slow vibrations or long waves; a big self-induction and 
capacity were necessary, and the arrangement was not applicable to the extremely 
high frequency needed for an effective sending station. 
Poulsen, however, improved the arc by immersing it in hydrogen gas under various 
pressures, until he had got an arc of really high frequency, which he patented in 1903, 
and thereafter used it for generating continuous waves of a frequency such as would 
generate radiation. 
Then came the question of receiving such waves. The thermionic valve was 
known as a rectifier, that is to say, it transmitted a current in only one direction, 
because it conveyed the current by the flight of electrons in a partial vacuum. Every 
vacuum tube acts as a rectifier, since it only transmits the current in one direction. 
Fleming perceived that this rectifying action of a partial vacuum could be employed 
as a detector of ether waves, and so patented the vacuum valve as a detector in 1904. 
It was used at first for the discontinuous system of spark signalling. The unrectified 
pulses were far too rapid to affect a galvanometer, or even a telephone; but when 
rectified, so that only half of each wave was employed, tke pulses could act in groups, 
and each group could cause a sound in the telephone, so that if the groups followed 
one another in regular succession they could cause a musical note whose pitch depended 
on the frequency with which the groups succeeded one another. 
To get the groups out of the continuous wave Fessenden devised the heterodyning 
system of receiving, that is to say, he superposed on the received wave another wave 
of nearly the same frequency, so that it would beat with the first. The actual vibra- 
tions of neither wave can be heard, but the beats, which represent the difference of 
the frequencies, are much slower than the generating waves, and therefore come 
within the range of audition. The beats may, for instance, succeed one another at a 
thousand a second, whereas the generating waves had a frequency of a million. By 
this combined plan of heterodyning and rectifying a modulated continuous waye 
to be received, no matter how complex the modulations were, it was possible to 
superpose upon a carrier wave the modulations of a human voice applied to it by a 
microphone ; and then these complicated modulations could be received at the 
distant station, and the tones of voice reproduced. This was the beginning of wireless 
telephony, which depends upon emitting and receiving the modulations of a continuous 
wave. 
But the amount of energy received at a distant station was small, and accordingly 
the voice was very feeble. But De Forest introduced a grid into the valve, giving 
it three electrodes instead of two, and supplied the valve with a local high tension 
battery, so that a stream of electrons travel from cathode to anode, passing the grid 
on the way. If the grid was now supplied with the pulsations of the received wave 
it would sometimes be positive and sometimes negative in potential. When it was 
positive it would help the electrons up on their way ; when it was negative it would 
repel them and beat them down. Thus it stood in the middle of the traffic like a 
policeman and regulated it, sometimes helping it on, sometimes stopping it. But 
the energy of the traffic does not depend on the policeman ; he merely regulates it. 
So it is with the grid. The energy of the triode valve is determined by the local 
battery, and may have any value you please. But the regulation of it, that is, the 
ee 
