Improvements relating to the Transmission of Electric Currents and to Conductors therefor.
Year: 1894

page_text: Date of Application, 2nd Jan., 1894-Accepted, 10th Feb., 1894
COMPLETE SPECIFICATION.
Improvements relating to the Transmission of Electric Currents and
to Conductors therefor.
I, MICHAEL IDVORSKY PUPIN, of No. 46 West 72nd Street, in the City of
New York, in the County of New York, and State of New York, United States
of America, Electrician, do hereby declare the nature of this invention and
in what manner the same is to be performed to be particularly described and
ascertained in and by the following statement :-
The invention relates to electrical transmission principally for telegraphic or
telephonic purposes.
The present limit of the rapidity with which telegraphic messages can be sent
over submarine cables, is very low, and, on the other hand, the transmission of
telephone currents over submarine cables of considerable length, say over 50 miles,
is practically impossible. It is well understood that the difficulty of transmission
of alternating, intermittent, or any other kind of variable currents over long
electrical conductors, such as submarine cables, is owing to the physical fact that
the combined action of the self-induction, the electrostatic capacity, and the
electrostatic absorption of the insulation of such conductors, when constructed
according to methods which now prevail, offer an insurmountable barrier to these
currents.
My investigations show that alternating, intermittent, or varying currents of any
kind, even when their rate of variation or frequency is high, can be easily
transmitted over conductors of great length, as, for instance, a transatlantic cable.
I will first state the physical facts and principle underlying my invention.
1st physical fact. Every electrical current behaves, in consequence of its self-
induction and capacity towards a periodically varying electromotive force, just as
a heavy elastic body, in consequence of its inertia and elasticity, behaves towards
a periodically varying disturbance. For just as such a body has a definite period of
vibration, so au electrical circuit has a definite period of its own, that is to say,
when its electrical equilibrium is disturbed by an external impulse a periodic
electrical current will result, the period of said electrical current depending or
the self-induction, the electrostatic capacity, the ohmic and other frictional
resistances of the circuit, like magnetic and dielectric hysteresis. The ohmic and
other frictional resistances can generally be reduced in such a way that they will
practically have no influence on the period of the circuit. In such cases, the
period depends on the self-induction and the electrostatic capacity of the circuit,
so that, by a suitable change of these two, the periodicity of the circuit can be
changed in any way that may be desirable. The process of changing the period
of a circuit by a suitable change in either its coefficient of self-induction, or in its
electrostatic capacity, or in both, I shall call "tuning the circuit."
2nd physical fact. The impedance which such an electrical circuit offers to an
impressed simple harmonic electromotive force diminishes gradually with the
approach of the periodicity or pitch of this electromotive force to the
periodicity or pitch of the circuit. It is a minimum, and equals the ohmic
resistance of the circuit when the two periodicities are the same, that is
when the impressed electromotive force and the circuit are in resonance. If a
complex harmonic electromotive force acts upon such a circuit and if the
periodicity of the circuit is considerably above the highest periodicity of the
harmonics contained in the complex electromotive force, then the impedance which
the circuit will offer to the various component harmonics of the electromotive
force will be very nearly inversely proportional to the periodicity of these
components.

page_text: These two physical facts have been theoretically deduced and experimentally
verified by myself.
If, therefore, I can regulate the periodicity of an electrical circuit, I shall be
able to regulate the impedance of that circuit to electromotive forces of various
frequencies; and I shall also be able to regulate by it the time constant and the
electrical absorption in the insulation of the circuit. This I do as follows:
I divide an electrical circuit into any number of parts. If these parts act upon
each other by electrodynamic induction, or by electrostatic induction, or by both,
then the period of the total circuit is equal to the period of any one of its parts,
if said parts are equal and essentially similar. Hence, according to the 1st physical
fact, if the ohmic and other frictional resistances in such a circuit, due to dielectric
and magnetic hysteresis, are reduced in each part in such a way that the period of
any part depends on the self-induction and the electrostatic capacity of that part,
then the period and the time constant of the whole circuit, no matter what its
length may be, will depend on the self-induction and the electrostatic capacity of
any one of the parts, and on nothing else but if said parts are not equal nor
essentially similar, then the circuit may be said to contain as many periods and
time constants as it has parts. I call these the partial periods and partial time
constants of the circuit. There will, however, be one period and one time constant
for which the impedance of the circuit is a minimum. This period I call the
period of the circuit; it lies between the shortest and the longest partial periods.
Then the condition of resistances above mentioned, being fulfilled, both the
partial periods and the resultant period will depend on the coefficient of self-
induction and the electrostatic capacity of the various parts, and on nothing else.
The same is true of the time constant.
Let, therefore, a long electrical cable be divided into any number of parts, and
let condensers be interposed between these parts, so that the various parts of the
cable connect the various condensers in series. It is evident that this division
_.divides the total static capacity of the cable itself into parts, and when the
condensers are interposed in the manner just mentioned, then we have a series
connection not only between the static capacities of the several condensers, but also
between the electrostatic capacities of the various parts of the cable itself. The
effective capacity of the cable can thus be reduced to any desirable limit.
To illustrate this, consider a cable 6000 miles long, having a total electrostatic
capacity of 500 microfarads. Dividing it into 6000 equal parts, making thus each
part one mile long, we shall have an electrostatic capacity of microfarad per
mile. Interposing 6000 condensers of 20 microfarads each, we shall have for the
total effective electrostatic capacity of the cable 300+ 72000 microfarads. That
is, we shall have reduced the effective electrostatic capacity of the cable to
nearly rado part of its original value before division, and we shall also have the
line capacity of the cable very small in comparision to the electrostatic capacity of
the interposed condensers. Therefore, I may construct the cable so that the
statical charge of the line, at any moment during the variable flow of an electrical
current along the cable, can be made small in comparison to the static charge in
the interposed condensers; hence the electrostatic absorption of the whole cable can
be made practically independent of the dielectric properties of the insulation of the
cable, and be confined entirely to the dielectric properties of the insulator employed
in the interposed condensers. Hence, if condensers possessing no electrostatic
absorption are used, as, for instance, carefully constructed mica condensers, then
the electrostatic absorption of the total cable can be reduced to any practical limit
that may be desired. There is an additional advantage gained by this subdivision
of the line capacity which I shall discuss later on.
Referring now to the accompanying drawings,
Fig. 1 is an electrical diagram symbolically representing a conductor divided
into parts and having interposed condensers, constructed and arranged in accordance
with my method.

page_text: Similar letters of reference indicate like parts.
Suppose it is required to construct a cable to connect two places 3000 miles
distant from each other, the cable to have 6 thousand ohms resistance and a
periodicity of 1800 periods per second.
Fig. indicates a preferable construction of a part, X Y, of such a cable,
according to my method. The cable is divided into equal sections, A, B, C, D, E,
and the return sections, A¹, B, C, D¹, E'. A A', B B', &c. act upon each other
through electrostatic induction by means of the condensers a a, b b, c c, d d¹, &c.,
or in other words, the condensers are connected in series to each other by means of
the sections of the cable. Let the capacity of each condenser be 20 microfarads,
and let there be 3000 pairs of condensers so that two successive pairs are at the
distance of one mile from each other. The sections A A', B B¹, &c., will therefore
be each one mile long. To simplify the description, I shall assume that each of
these sections consists of a thick cylindrical copper wire 0.5 cm. in diam. It is
evident, however, that wire ropes or flat strips of a conductor can be used as well.
Well annealed copper being used we shall have for each mile about 1 ohm
resistance. Hence for 6000 miles 6 thousand ohms. Let the adjacent cylindrical
conductors be parallel to each other, and let the distance between their axes be
2.72 multiplied by their radius. We then shall then have for the self-induction
of the conductor connecting two successive condensers, denoting it by L, L
⚫000396 Henrys.
=
Since each conductor is connected to a capacity of 20 microfarads, we shall have
where T is the period of any one of the equal parts of the cable, measured in
seconds, L, its self-induction, measured in Henrys, and C, its capacity, measured
in microfarads.
The resistance has a very small effect upon the period, as a simple calculation
will show;
To simplify my explanation, I shall first suppose that the line capacity of any
one section of the cable is infinitely small in comparison to the capacity of the
terminal condensers of that section.
The impedance of this cable to a simple harmonic current of 1800 periods would
be 6000 ohms, that is to say, a simple harmonic electromotive force whose period
is 180g of a second and whose mean amplitude is say. 1000 volts, would send
through such a cable a current whose mean value will be ampere reversing
3000 times per second.
This form of cable I call a high pitch cable, because it has a high periodicity,
much higher than the periodicity of the notes in the average human speaking voice.
It is evident that a periodicity of 200 complete pediods per second would more
than suffice for telegraphic purposes, in which case it would be necessary to have a
much smaller number of 20 microfarad condensers, which would, of course, reduce
the cost of the cable very much. The high pitch cable, however, which I am
describing would be a form of cable suitable to the transmission of telephone
currents, for reasons which can now be stated briefly.
The range of notes in the average human speaking voice is about between
120 and 600 complete periods per second. To show the advantage of this form of
cable it is well to calculate the impedance for periodicities lying between the two
limits just mentioned. Let 120 stand for impedance to an electromotive force of
periodicity 120, and let coo stand for the impedance to an electromotive force of
periodicity 600, then

page_text: per second. To illustrate this result, I now assume that two suitable telephones, GH,
one at the station X and one at the station Y, 3000 miles distant from X, form a
part of this circuit, and that a note is sung into the telephone at X. Supposing
that the note is composed of a fundamental note having 120 periods per second
and of four upper harmonics having 240, 360, 480 and 600 periods per second.
It is well known that the vibrating telephone diaphragm at X will induce in the
circuit a complex electromotive force consisting of the fundamental simple
harmonic electromotive force having 120 periods per second and also of the upper
harmonics having the periodicities of 240, 360, 480, and 600 periods per second.
A perfect telephone diaphragm would produce these harmonic electromotive forces
in such a way, that their amplitudes would be in the same ratio to each other as
the amplitudes of the sound harmonics in the note which is uttered into the
telephone. If, however, the diaphragm of the telephones G and I are of low
pitch, then the induced electromotive force would be stronger in its lower harmonics
than the sound waves which set the diaphragmn into vibration; it is evident,
therefore, that the tendency of the telephone diaphragm to strengthen the lower
harmonics of the sound can be counterbalanced as nearly as desirable by the
tendency of the cable to strengthen the upper harmonics; so that a long distance
transmission of telephonic currents by a cable of the description just given is not
necessarily accompanied by a distortion of the sounds transmitted.
I assume now that the mean value of the induced simple harmonic component.
electromotive forces with a perfect telephone diaphragm would be

page_text: 1
That is to say, the itensities of the component simple harmonic currents would be
to each other in the same ratio as the intensities of the simple harmonic sounds of
which the sound uttered into the telephone is composed. Hence there would be no
discoloration in the transmitted sound. It should be observed here that every one
of the above currents is very much stronger than the currents which can produce
audible sounds in the telephone, for, according to trustworthy investigations,
currents of the order of magnitude of 1013 amperes produce sounds in the telephone
that can be heard very easily. But these numerical values of the currents just
obtained will be modified if the hypothesis on which the calculations so far have
been made is dropped, the hypothesis, namely, that the electrostatic capacity of the
conductor of any one section of the cable is infinitely small in comparison to the
capacity of the terminal condensers of that sectior A brief discussion of that
modification will bring out clearly another very great improvement which I can
obtain by constructing a cable according to the method described in these
Specifications.
-
Consider the instantaneous electrical state of the cable at any moment when an
alternating current is flowing through it. Commence at the first pair of condensers
nearest to the transmitter. One condenser will be on the positive side of the
transmitter at that moment and the other on the negative side. I shall confine
now my attention to the series of condensers which are at the moment considered
on the positive side of the transmitter. This series consists of 3000 condensers
distributed equidistantly between the two stations. One side of the first condenser
of the series will have, at the particular moment under consideration, a charge m.
This charge induces a charge m₁ on the opposite side of the condenser. In the
second condenser, we have charges + m, and m2, and so forth until we reach
the last condenser. In this one we have charges m₁-1 and
An exactly
similar distribution of charges will take place in the series of condensers on the
negative side of the transmitter. In my case n = 3000. The charges m and m₁,
m₁ and ma, etc., are not numerically equal to each other because the capacities of
the conductors connecting the various condensers are not infinitely small in
comparison to the capacities of the condensers. Let the capacity of half the
conductor connecting two consecutive condensers be the 7th part of the capacity of
one of the condensers-then we shall have
-
-
ma'
The currents in the various sections of the cable would vary in very nearly the
sanie ratio, that is to say, the value of the current at the receiving station would be
only the 5 part of the theoretical value, deduced on the supposition that the line
capacity of the conductors of the various sections is infinitely small in comparison
to the capacity of the condensers. To show now the numerical value of these
currents at the receiving station, I shall point out the value of the weakest one
among them. It will be

page_text: As observed before, this current is, according to very trustworthy investigations'
over a thousand times stronger than the weakest current that can be heard in a
telephone. It should be also remarked that the telephonic current which could be
sent over the cable just described would be much greater, because the impressed
electromotive force generated by the induction coil of a good transmitter is much
greater than the voltage with which I started in the above calculations.
To state, in a concise form, this very important improvement in cables constructed
according to my method, I introduce now the following definition:-Consider the
current in the conductor of any section of the cable. That current consists of two
distinct parts. One part is due to the charging of the terminal condensers of that
section, and the other is due to the charging of the surface of the conductor itself,
The first current I call the longitudinal displacement component, and the second
current I call the lateral displacement component of the current in that section.
The above-mentioned very important improvement consists, therefore, in diminishing
the ratio between the lateral and the longitudinal displacement components of a
rapidly varying current. The wasting of a current due to the lateral displacement
components in the various parts of a cable acts evidently like an impedance, and
I wish it, therefore, to be distinctly understood that in my claims the word impedance
of the cable contains the impedance due not only to ohmic resistance, self-induction,
etc., but also the impedance resulting from the lateral displacement currents.
A variety of designs of such cables is readily suggested by the fundamental
physical facts noted, and need not be enumerated here.
Referring now to Fig. 2: for the purpose of giving the divided cable an
uninterrupted metallic circuit so that in case of a break the fault may be located by
ordinary methods, there is connected with each condenser in shunt a coil, as m m²,
n n', of many turns, surrounding a closed or substantially closed magnetic circuit,
such as an iron ring, r. These coils, in consequence of their large self-induction and
also in consequence of the electro-magnetic property will not allow an appreciable part
of the main current to pass from one section of the cable to the next as long as the
frequency of that main current is anywhere above 50 periods per second. It has
long been known that when a condenser is connected in shunt with a coil and an
electromotive force is used having a periodicity somewhere about that of the
periodicity of the circuit composed of the condenser and coil, that then considerable
current will pass through the coil even if the self-induction of the coil is very large;
but this relation holds true, as I have experimentally discovered, only when the
iron core of the coil is exceedingly well laminated and does not form a closed
magnetic circuit. But if the magnetic circuit is closed, then this relation is far
from the truth. The mean value of the current that will pass then from one section
of the cable to the next through any one of the auxiliary coils is equal to the mean
value of the difference of potential in the condenser which is shunted by that coil
divided by the impedance which the self-induction and the obmic resistance offer
to this difference of potential. Making, therefore, this impedance large, the leakage
current can be reduced to any limit, especially for frequencies above 50 periods per
second. In other words, the auxiliary coils act somewhat like electromagnetic
valves.
The effect is that, so long as the condensers remain in order, only an inappreciable
part of the said current will pass through the auxiliary coils, and hence the coils
will not exercise any material influence upon the results before noted; but if a
condenser should, from any cause, become inoperative, then this will be at once
shown by the increase in the impedance of the line,-but as the line remains
continuous, it is possible to locate the fault by determining the capacity of the
cable, or by other suitable means which might not be applicable if the sections of
the line had no metallic connection between them. Or, if the cable should be
broken, in which case it would become inoperative, the fault may be located by
ordinary methods applicable to metallic lines.
If the sections of the cable, instead of being equal and similar, as above described,
were made unequal and dissimilar, then substantially the same results would be
5

page_text: obtained. We should simply deal with the mean periodicity of the line, and
establish it with reference to some previously selected periodicity, which, in the case
of telephonic transmission, we should make preferably higher than that of the
highest strong note of the voice.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed, I declare that what I
claim is;-
1. Overcoming the impedance which long electrical circuits possessing considerable
self-induction, static capacity, electrostatic absorption and large time constant offer
to variable currents by tuning such a circuit so as to give to it a previously selected
periodicity by dividing said circuit into a number of parts, preferably equal and
similar, and bringing said parts into inductive relation to each other either by
electrodynamic or electrostatic induction, or both.
2. The construction and arrangement of an electrical conductor composed of
parts or sections placed in series and provided with condensers in circuit with and
interposed between said sections, a coil in shunt with each condenser and a closed
or substantially closed magnetic circuit surrounded by said coil.
Dated this 2nd day of January 1894.
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C., Agents for the Applicant.