New-York tribune., 1922-10-01
Elusive Action
OF Negative
Resistance
Term Made Prominent by
Invention of the Super
Regenerative Circuit
Explained by Analogy
ConditionReversedinV.T It.
Resistance More Important
in Radio Sets Than 1t
Is in Electric Circuits
By Ralph X. Potter
Tribune Institute E1gineer

Super-regeneration has introduced O
number of new terms for the radio
enthusiast to struggle with. Amone
these is negative resistance" which
plays an important part in the proper
operation of Major Armstrong's latest
radio receiver.
Resistance Of substance, as ordi-
narlly met with in electric circuits,
is a measure of it's ability to obstruct
current flow. Thus we consider apcn-
Gil line along strip Of cardboard (as
used for Ii grid leak) to have a very
high resistance because a relatively
high voltage source impressed across
1t results in the flow Of only an ex-
tremely small current.
A resistance of this type is meas
ured In megohms, or millions of ohms,
while that of a piece of copper wire
the same length might be only thou
sandth part of an ohm or less. In an
electrical lighting system materials
with resistances that are measurable
in megohms fall in the class of insu-
lators, but in radio they may perform
very important function in the con-
ducting circuit. A year ago such a
ohihB t1S a lieBoLlH resistance Woulu
have been classed among the "high-
brow" devices of thick-spectacled
scientists by the maiority of us who
to-day find enjoyment in straining
music out of the air.
Water Pipe Analogy
Any enthusiast with enough
pioneering spirit to "roll his own" in
the way of vacuum tube receiving
set has by this time added the
"megohm" term to his radio vocabu-
larry. Applying the hydraulic analogy
to resistance a long line of pipe car
TO the internal resistance of this pipe
the flow will be limited by the pump
pressure at the source-and the higher
the pump pressure the more viii be
the delivery at the other end. Ordi-
nary resistance in electrical conduc-
tors acts similarly-that is, the more
electrical pressure in volts there is
over the conductors length the
greater will be the current flow.
There is, however, another type OF
resistance that falls somewhat behind
the range of the original significance
we have attached to the term. This is
known as negative resistance and 1t IS
particularly important in the action OF
the vacuum tube. When the tube fila
ment is aglow and positive potential
impressed upon the plate (by attach
ing the "B" battery across from fila
ment to plate) there is flow of mega
tive charges or electrons across the
intervening space from the former to
the latter. A variation of this electron
flow is produced by the varying charge
on the grid between the two--a mega
tive charge on the grid retardng the
progress of the electrons and posi-
tive one assisting them in their move
ment.
Reversed Conditions
Contrary to experience with ordi-
nary resistances, in which current in
creases with increase in potential, cur
rent and potential in the flament-to-
plate circuit vary in the opposite di-
rection, the potential actually dccreas-
ing as the current flow increases. Due
to this reversal Of conditions as met
with in the usual current flow prob
lens, the term negative" has been
prefxed as means of designation.
To overcome resistance requires an
expenditure of energy, whether con-
sidered from the electrical or mechani-
cal point of view. Dragging sled on
the bare pavement will convince most
Of us of the mechanical application.
On the electrical side are such ex-
amples as the light bulb that radiates
the energy used up in overcoming it's
resistance in the form of light and
heat, the coil heater dissipating energy
largely as heat alone, and (taking a
long jump to radio circuits) the trans
mitting antenna that broadcasts energy
in the form of electromagnetic waves.
Negative Resistance Explained
These illustrate the so-called posi-
tive resistance circuits. Now # it
were possible to put as much electrical
energy back into circuit of the posi-
tive resistance type as was being radi-
ated from it the system would, accord
ing to electrical measurements, have
no resistance at all, because apparently
no energy was being used up in NOV
ing the current along it's path And
If more energy is supplied to this cir
cult than is being radiated there will,
according to the meters, be less than
mo resistance at ALL" SO that in order
to classify it's resistance we will be
forced to call it negative Following
this definition, any circuit has a mega
tive resistance between it's two termi-
nails when, between those terminals,
hioie SueiSy iS DelHS absorbed than
radiated.
Negative resistance is by no means
always present in the filament-to-pla
circuit of vacuum tube. 1t is pro-
duced only with definite relation be-
tween the plate and grid circuits, as.
for example, is used in the arrange
ment for the oscillating tube of the
per-regenerative receiver When the
Csclllator refuses to function II is a
sign that negative resistance is not
being produced by the feed-bac of en-
ergy from the output to the input or
from plate to grid circuits. Just as
soon as the proper coupling IS made
the h,ament-.to- plate path develops it's
negative resistance reaction character
lstic and oscillations start. the excegs
OF energy absorbed {according to the
above definition) being supplied by the
'B" battery in the plate circuit,