Improvements in or relating to Apparatus for Indicating the Strength of Current upon an Electric Circuit.
Year: 1888

page_text: Compitie opornication,
COMPLETE SPECIFICATION.
Improvements in or relating to Apparatus for Indicating the Strength
of Current upon an Electric Circuit.
I, OLIVER BLACKBURN SHALLENBERGER, of Rochester, in the County of Beaver,
and State of Pennsylvania, U.S.A., Electrical Engineer, 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 the class of apparatus employed for indicating the strength
of current upon an electric circuit.
The object of the invention is to provide convenient, effective and reliable means
for determining the difference of potential existing between two given points in an
electric circuit supplied with alternating, intermittent or pulsatory electric currents.
It is well known that the value of a given electric current is proportional to the
electromotive force to which it is due, divided by the resistance which it encounters.
If, therefore, the resistance is decreased at the same time that the electromotive
force is increased or vice versa, then the current flowing will increase or decrease at
a rate greater than the increase or decrease of the electromotive force. The present
invention aims to substitute for such variable resistances a variable counter-electro-
motive force, the amount of which is dependent upon the variations in the difference
of potential to be determined. A circuit having an appreciable self-induction, when
traversed by alternating, intermittent or pulsatory electric currents opposes a counter-
electromotive force varying in the manner required, for this purpose, that is to say,
as the current traversing it decreases, the apparent resistance due to self-induction,
diminishes, and vice versa, although the actual specific resistance remains constant.
It is possible to give to inductive devices of different forms or proportions, different
rates of increase or decrease of current under the influence of the same variations in
the difference of potential, and therefore a current traversing two such inductive
resistances in series or parallel will produce a combined effect different from that in
either acting separately.
Two general plans of employing a self induction device or reactive coil will be
described, one plan in connection with figures 2, 3 and 4, and the other in connection
with figures 5 and 6.
In connection with figure 1 the law of increase of current will be described.
In this figure the vertical axis 0-Y is taken as the direction in which the current
is measured, and the horizontal axis 0-X the corresponding values of electromotive
force. By Ohm's law it is known that the value of a given current is always propor-
tional to the electromotive force, the resistance being constant. The law of increase
of current through such a resistance is therefore represented by a straight line O-A.
But if the resistance from any cause decreases as the electromotive force increases,
then the current will increase in a greater ratio, and their relation will be indicated
by a curved line such as 0-B. Circuits may, however, be so organized that their
apparent resistance will change at different rates. Thus another circuit might be
caused to decrease its resistance at such a rate that the relation of the current to the
electromotive force would be indicated by the line O-C.

page_text: A coil of wire provided with an iron core has the quality of opposing to an alter-
nating or variable current, a counter electromotive force or apparent resistance much
greater than that due to the actual resistance of the conductor itself. This apparent
resistance does not with a varying current bear a fixed ratio to the actual resistance,
and may in coils of different forms follow different laws of variation as represented
by the lines 0-B and O-C. By suitably proportioning two coils, the equal
currents may traverse them at any given electromotive force, as indicated at 0-a, as
shown by the intersection of 0-B and 0-0 at v, while at any value of electro-
motive force greater or less than 0- the currents are unequal. The same may be
said of a simple resistance having the law shown at O-A and either of the coils
following the law 0-B or 0-C.
Referring now to figure 2, A¹ represents a suitable source of alternating, inter-
mittent or pulsatory electric currents; L', L2, the main lines leading to any
suitable work circuit. Between the lines L' and L2 there is connected the
indicating apparatus to be described. This consists in this instance of an indicator D¹,
here shown as consisting of a lever d' carrying a pointer e applied to a scale-
plate f¹. From one end of the beam d' there is suspended a core g¹ extending
into a solenoid. A counterpoise ¹ is suspended from the opposite end of the
beam d¹.
The solenoid is included in a conductor extending from the line L¹ to the
line L², being connected there with at points 1 and 2.
A coil M² of high self-induction is included in series with the coil . This
coil is provided with a core m' of soft-iron. This core as also the core g¹ is
preferably made up of fine soft iron wires or laminae of soft iron.
An adjustable resistance M is also included in series with the coils M¹ and h².
The method of determining the electromotive force is simply to establish a given
normal current through the coil M of high self-induction, by placing a fixed
difference of potential upon it, and noting the amount of resistance which must be
introduced into the circuit in order to establish this normal difference of potential.
That is to say, the instrument will only balance, bringing the pointer at the center
of the plate f, when a certain predetermined difference of potential is maintained
between the points 3 and 4, upon the opposite sides of the coil M₁ and solenoid h¹;
the additional difference of potential existing at the terminals of the resistance M
being determined by the position of the indicator n¹ upon the scale n.
The resistance M may be included in shunt upon the self-inductor M¹, as
shown in dotted lines, in some instances.
If the rate of alternations of, or periodic changes in, the current should remain
approximately the same, no adjustment of the resistance M is necessary in order to
insure the accurate operation of this instrument. This condition is fulfiled in those
systems of electric distribution where all the machines are intended to run at
approximately the same number of alternations per minute, say 16,000. But where
there are slight variations in. the number of alternations of the current, the self-
induction will vary with the rate of alternation as well as with the electromotive force,
but the adjustable resistance device M may be used as a correction for the slight
variations thus occasioned. The error due to this cause is usually very small, but in
some instances this adjustment may be desirable.
In figure 3 a modification is shown wherein the amount of self-induction in the
circuit can be varied. This can be accomplished by moving the core m¹ of the self-
induction device M³, within its coil, an indicator o being applied to a scale o¹ and
moving with the core. The resistance M and device D¹ are included in the circuit
as before. In this instance, a converter C is interposed between the lines 1, 1, and
12, 12, the former being connected from the line to the line L², through the
primary coil p of the converter. The conductor 12 is connected through the
secondary coils, and the devices M³, M and D¹. The converter serves to reduce
the current in any required proportion so as to make it more convenient for
handling.

page_text: The adjustment of the device M' serves also to correct for the variations in the rate
of alternations, if it is so desired.
In figure 4 there is shown an organization by means of which slight changes in the
rate of alternation may be compensated for automatically. Two self-induction
devices M² and M² of different forms are employed, and they may be so proportioned
that, while an increase in the number of alternations may increase the self-induction
of one it will have the opposite effect upon the other. It is a well known property of
coils possessing a self-induction in which an iron core is present, that there is a par-
ticular point at which the self-induction remains approximately constant through a
considerable change in the rate of alternations, while it decreases both above and
below this point. It is therefore necessary to suitably proportion the two coils and
use one above its particular approximately constant point, and the other below such
point, in order to compensate, within certain limits, for the error due to changes in
the rate of alternation. The coil of the indicating apparatus may be utilized in
certain cases for this purpose, by making it of suitable form and proportion, thus
dispensing with the use of a separate compensating coil.
The two coils M' and M² might be connected in parallel with each other in the
same manner as the coil M¹ and the resistance M. in figure 1.
In figure 5 the conductor is divided, between points 3 and 4 into branch
conductors and 12. The branch contains a self-induction device M¹ consisting
of a coil of wire surrounding a core m¹; it also includes one coil applied to an
indicator D. The branch contains a constant resistance R¹ which may be adjust-
able, and also a second coil h¹ of the indicator D¹. The coils and h¹ are provided
with cores j and g¹, preferably of soft iron wires insulated from each other, and they
are respectively supported upon opposite ends of a beam d¹. This carries the pointer e¹
applied to the scale f¹.
An adjustable resistance M may be included in the conductor between the points 1
and 3 and this is here shown as provided with a scale n and indicator n¹.
A determined difference of potential is produced between the points 3 and 4, and
the difference of potential between the points 1 and 2 may be determined by adding
the difference of potential at the terminals of the resistance M, which is known by the
scale. For this purpose the resistance M has been previously measured and calibrated,
so that by noting the position of the indicator, the resistance in the circuit may be
determined and thus the difference of potential which will exist at its terminals may
be known. If the electromotive force rises beyond the predetermined point, then the
increase of difference of potential between the points 3 and will cause the apparent
resistance of the self inductor M¹ to be decreased, thus causing a greater proportion of
current to traverse the branch . This, acting through the solenoid will move the
indicator e in a given direction. If, however, the difference of potential falls below
the predetermined point, then the apparent resistance of the self-induction device will
be greater than that of the resistance R¹, and a greater proportion of current will
traverse the branch and move the indicator in the opposite direction. By thus
adjusting the resistance M until the indicator e¹ comes to its zero point, the difference
of potential existing between the points 1 and 2 may be readily indicated.
In figure 6 a slightly different organization is shown. In this instance, two self-
induction devices M1 and M2 are employed. In this instance, the device M¹ is made
adjustable, having a pointer n' applied to a scale n. The two self-induction devices
are of different form and so proportioned that their self-inductions will vary in
different ratios, or they have a different law of change under the same circumstances.
Thus the device M' may have the law of change represented in the diagram figure 1,
by the line 0-B, while the device M² varies according to the line 0-C. By
adjusting the core m¹ the balance under the influence of the required difference of
potential at the points 3 and 4 may be obtained. Then an increase of difference of
potential will cause one of the devices to decrease its apparent resistance more rapidly
than the other and likewise a decrease of difference of potential will cause it to increase
its apparent resistance more rapidly than the other. If the difference of potential
between the points 3 and 4, which is required to obtain the balance of the indicator D¹,

page_text: is known, then in case of a variation in the difference of potential, such increase or
decrease may be determined by adjusting the core m³ until the balance is restored and
noting the change upon the scale n.
In this instance the coils h¹ and
are shown as applied to a single core g¹, being
wound in opposite directions and therefore having a differential action thereon. The
weight of the core is balanced by a weight i.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed I declare that what I
claim is :-
1. The combination of an electrical indicator for alternating, intermittent or pulsatory
electric currents, and a conductor of high self-inductive qualities, whose coefficient of
self-induction decreases with an increase of applied potential, in circuit with such
indicator, whereby the sensitiveness of the instrument is increased.
2. An electric indicator for alternating, intermittent or pulsatory electric currents
consisting of an indicating device affected by such currents, and two or more self-
inductive devices having different coefficients of self-induction in circuit therewith.
3. The herein before described method of determining the variations in the difference
of potential between two points in an electric circuit for alternating, intermittent or
pulsatory electric currents, which consists in establishing in the circuit connecting said
points, a high self-induction and indicating the variations of current passing through
such circuit of high self-induction, substantially as described.
4. The combination of an electric conductor, branch circuits for the same, a self-
induction device in one branch, a resistance included in the other branch, opposing
solenoids included in the respective branches, and an indicator acted upon by
alternating, intermittent or pulsatory electric currents traversing said solenoids.
5. An electric indicator for alternating, intermittent or pulsatory electric currents
consisting of two branch circuits, a self-induction device in one branch, means for
balancing the normal apparent resistance of the self-induction device in the other
branch, two opposing solenoids in the respective branches, an indicator to which said
solenoids are applied, and an adjustable resistance connected in the conductor leading
to said branch circuits.
6. An electrical indicator for alternating, intermittent or pulsatory electric currents,
consisting of an indicating device having differential coils, and two reactive coils of
unlike self-induction connected in circuit with the respective coils of the indicator.
Dated this 4th day of August 1888.
WM. P. THOMPSON & Co.,
Agents for the Applicant.