New-York tribune., 1922-09-24
BroadcastMade
p os sible By
Vacuum Tube
Steam Engine Analogy Shows
How 1t Gives Off the
Oscillations Needed to
Produce Carrier Waves
Latter Travel Thru Space
Carrying With Them Audi
bic Currents Correspond
ing to Music and Speech
BY Ralph K. Potter
Tribune Institute Enqineer

Tribune Institute Engineer
a great deal of the progress that has
been made in the broadcasting and re-
ception of music and speech by radio
IS dependent upon the fact that a
three-electrode vacuum tube is capable
Of generating what is known as an
oscillating current. This oscillating
current consists primarily OF a direct
current, the rate OF flow of which is
periodically increased and decreased.
1t must not be confused with alternat-
ing current. such as is used in many
parts of the city to operate the electric
lighting systems. Alternating current
flows in one direction for part of its
cycle and then changes direction COM
pletely. Such a reversal of current in
the vacuum tube is impossible, due TO
it's valvelike character, since it per-
mits a flow in one direction only.
The oscillatoxy characteristics of the
threc-electrode vacuum tube have been
Of great value in producing the ex-
tremely high frequency carrier cur
rents that create the electromagnetic
waves radiated from a broadcasting
station. These waves are the carriers
that transfer music and speech through
space to the aerials of the receiving
sets by changes in their successive
ampIitudes corresponding to variations
IN the tone impressed upon the trans
mitter at the broadcasting station.
When we speak of a 360-meter wave
We mean that the electro magnetic i'm
pulses are being propagated at the rate
Of almost o million a second The
problem of constructing a rotating
machine to generate an alternating
current of this frequency is prodigious
and the three-electrode vacuum tube is
an expedient which performs the task
in q simple manner.
Aids Continuous Wave Reception
The highly selective and efficient
hetrodyne system of continuous wave
code reception is another example Of
the oscillating tubes application. BY
super-imposing the oscillations OF u
loeal circuit upon those of the incom-
ing signal, it was found possible TO
produce o resultant oscillation of a
frequency that was within the range Of
hearing. Those who have operated a
regeneratiye receiving set have no
doubt heard the musical note of con-
tinuous wave signals and have found
that the pitch changes considerably
with the slightest movement of the
tuning handles of their set. In this
case the receiving set is producing the
local oscillations necessary to give
audibility and the change in pitch is
caused by variation in the frequency
of these local oscillations.
Provided a regenerative set is prop
erly constructed 1t is capable OF pro.
ducing oscillations. As the ticker is
moved toward the tuning circuit, 1t is
common experience that the music OF
speech will be amplified until at a GER
tain point n squeai sets in. Further
movement Of the ticker results in :
complete drowning out of the incom-
ing signal, and if the operator wisho;
TO keep on speaking terms with his
netghbor, who has an aerial near Lv
he had better. decrease the ticklel
coupling promptly. The squeal means
that the tube is oscillating and undel
these conditions the receiving set he
comes low-power broadcasting sta
tion with nothing on its program ex
49, cept noise.
& Cured by Dose of Same Poison
# IN the super r-regenerative circuit. re
~ Gently introduced by Major Armstr
the osclilatlng tube is put to new ant
unique task. BY throttling the ten
dency of one tube to oscillate with tn,
osclIiatIons generated by another tube
thls circuit makes use of much close
ticker coupling, and consequently fa
greater signal amplifcation. than wa
possible with the original regenera
tive circuit. Thus an effective antidot
mas, IN the case of the regenerative cir
cut, proved to be a properly admin
istered dose of its own "poison"
The grid Of the three -electrode vac
uum tuoe acts somewhat as the trig
ger OF a gun releasing at a compare
lively small expenditure Of energy
surge OF electrons toward the plate
These eiectronS are minute electrica
charges and since the plate is charge,
positively by connecting the plus sid
OF the h battery to it there is a con
stant attraction of the eleetrons fror
the hot filament (where they are be
ing set free) to the plate surface.
positive charge on the grid assists tn
flow toward the plate while a negativ
charge retards it, but nevertheless tn
stream OF electrons is always movin,
toward the plate and away from thei
source, the fjlament.
Works Like Steam Engine
In the original model of the recjp
rocating steam engine the steam wa
turned on by the engineer movin,
a two-way valve, allowing the steam
TO so first into one end and then tn
other of the cylinder, making the mis-
ton move back and forth to turn tn
iywheel. Later the ingenious idea @
making the engine move the valy
lever controlling the steam flow to tn
cylinder was adopted with the resul
that the machine ran of its own ac
cord without the external" effort o
the operator IN moving the lever.
The thrce-electrode vacuum tube ca
be made to do the same thing by prope
coupling of the o1tput or plate cir
cult with that of the grid of inpu
side. When the ticker (Which make
such n coupling) is not coupled closel,
enough with the input inductanc
the result is similar to that in .
steam engine which cannot produc,
enough power to open and clos,
Sts own valves. A closer coupling give,
stronger impulses to the grid with tn,
result that oscillations start. Thes,
may be audible or not, depending upoT
the constants of the circuit, but pro
yided the circuit resistance is insult
fieient to damp them out they viii con.
nBue as long as the filament is re
leasing electrons