New-York tribune., 1922-08-27
hat Variation,
0f Resistance
Does i'm Set
Outlining the Positive and
Negative Resistance Of
Circuit Mentioned in
Armstrong's Invention
Action Highly Technical
When Power Leaves Circuit
Resistance Is Positive;
Negative When Entering


Ever since Major E. H. Armstrong
| made complete disclosure OF his in
vcntion tthe uper-rcgenerator) the
radio fans generally have been COM
pletely baffled by the terms negative"
and positive" resistance of the circuit.
In describing this system in his
technical paper before the Institute OF
Radio Engineers he said that there
we're three methods of producing the
regenerativc state by varying
the relation between the negative and
positive resistance Of the circuit. The
first method consisted of varying the
negative resistance with respect to the
positive and the second by varying the
positive resistance with respect TO the
negative, and the third, "a combination
OF those two systems in which stimuli
taneous variations are produced in
both negative and positive resistances
and provision made for adjusting the
relative phases of these two varia-
tions."
This description, of course, is too
highly technical to be readily under
stood, and it is very difficult matter
W uNocl lpC CiSuily JuSb WHaG is meant
by positive" and negative resist
ance Of circuit, but l shall try in
this article to make it as clear as l
possibly can.
Waves Are Alternating
In the first place, it must be under
stood that the electro magnetic waves
which travel from the transmitting
station to the receiving station are of
an alternating character. That is, they
change from positive to negative at
certain definite intervals each second
Another point that should also be
remembered is that the rapdity with
which these currents change from posi-
tive to negative is directly connected
in relationship to the length of the
wave that the transmitting station
sends out Thus, in the 360-meter wave
used by the majority of broadcasting
stations we have an alternating cur
rent which has a frequency of $4,000
there are that number of changes from
positive to negative every second.
The relationship between this FREE
quency of alienation and the length
of the wave is governed by the speed
with which electro magnetic waves
travel; namely, 900 million meters per
second. Therefore, it will be seen that
il a wave is 5000 meters long the
frequency will be 10000 cycles per
second.
Feedback Is Equal to Negative
The various types of receiving cir
cults are built to operate under the
conditions imposed by these frequcn-
cies. In the ordinary regenerative set
Armstrong states that the introduction
of fced-back coil is equivalent to
introducing a negative resistance in
the circuit, and this neutralizes the
positive resistance reaction of the cir
cult and thereby reduces the effective
resistance of the circuit.
Perhaps the simplest way in which
this rather complicated condition can
be explained is in the words of Pro-
fessor j. H. Morecroft, of Columbia
University, who says in his principles
of Radio Communication" that in an
alternating current circuit power must
be transferred between two points in
circuit in such a way that it may oe
either leaving the circuit OR entering
the circuit between those two points.
He then continues: IF power is
leaving the circuit between these
points, as heat or otherwise, the re-
sistance is positive; if power is enter
ing the circuit between these two
points the resistance is negative, and
If power is entering the circuit at the
same rate as it is leaving, then the re-
sistance is zero. From this standpoint
any electrical circuit carrying current
after reaching the steady state Ono
change in the amplitude of the cur
rent) has, on the whole, zero resist
ance, but we may consider the. source
OF power supply as having as much
negative resistance as the rest of the
circuit has positive resistance.
Relation of Two Resistances
AT the generator Cor other source of
power supply energy is entering the
circuit as fast as it is dissipated in
other parts of the circuit. If the cir
cult, as a whole, has positive rasis-
tance the current must be decreasing
in amplitude; this state of affairs oc-
curs in the ordinary damped oscila-
tory discharge of condenser whereas
a circuit which takes an appreciable
time to build up to it's steady state
! has. during the time required to reach
! the steady state, on the whole mega
| tive resistance because, considering
the circuit as a whole, energy is en-
| tcring at rate faster than that at
which energy is leaving.
/ In order to make this as clear as we
possibly can. we can momentarily as
4 sumo that the regenerative action is
/ equivalent to the negative resistance
| of the circuit, and that the damping
action which tends. to wipe out the
/ electrical v vibrations, corresponds to
| the negative resistance.
/ In the reeenerative receiver, the re-
! generative action will continue un TO
certain point when the vacuum tube
| SUDDENLY goes into state of oscilla-
tion and prevents Us from getting the
/ increased amplifcation that would
otherwise be possible.
What Armstrong has done, therefore,
! in this case. Is to devise system
which will wipe out the effect of damp
/ ing and permit the regeneration to
continue up to the limit OF the capacity
!! of the particular tube that is being
used in the circuit.
System Explained in Patent
| In the letters patent, which were is
| sued to him on July rs of this year, the
! system is described as follows "The
principle Of this invention may be un-
! derstood from the following general
| analysis. In the usual form of circuit
/ employing LIE regenerativs or feed
back principle, the regeneratiVe ampli-
/ fcation continuously increases as the
i feedhack coupling is Increased or
broadly as the amplitied oscillation Is
utilized moro and more to reinforce
| tho original oscillation, until the point
OF oscillation is reached; that Is. the
(, noir,t where the regeneyatiye effect is