Improvements in Apparatus for Finding the Range and Position of Distant Objects.
Year: 1890

page_text: Date of Application, 28th Aug., 1890-Accepted, 15th Nov., 1890
COMPLETE SPECIFICATION.
Improvements in Apparatus for Finding the Range and Position
of Distant Objects.
I, BRADLEY ALLAN FISKE of 1 W 81st Street City of New York, State of New
York United States of America, Lieutenant in the United States Navy, 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:
-
.
My invention relates to a method and apparatus for determining the distance or
range of far-off objects, and consists broadly in determining a fractional portion of a
conducting body bearing in length a ratio to the angle included between two lines of
sight directed upon a distant object, and simultaneously causing a disturbance in an
electrical balance including said body in its circuit, proportional to the resistance of
said fractional portion, and observing the difference in potential due to said dis-
turbance; and also in an apparatus whereby my said method is carried into practical
effect, which apparatus consists substantially in two bodies of conducting material.
similarly disposed with reference to a predetermined base line, alidade arms
establishing and maintaining contact with said bodies, and an electrical measuring
instrument constructed to measure differences in electrical potential, said instrument.
being provided with an index and a scale marked in units of linear measure, and
the aforesaid parts being connected in electrical circuit.
Referring to the drawing let AB be a base line, and T the position of a distant
object, the range of which AT is to be determined. By trigonometry, in the
triangle ATB, AT =
AB
sin ATB
==
xsin ATB. Let C and D represent two telescopes
or alidade arms pivoted at the points A and B and sweeping over arcs E and F of
conducting material; the said arcs having their extremities upon the base line AB.
Let the telescope C be directed upon the point T, assuming the position represented
by C, dotted lines. Then obviously, the angle C¹ AC is equal to the angle ATB and
the portion of the arc E included between the positions C and C of the telescope,
will measure the angle at ATB. In the foregoing formula, the base line AB is
known by measurement, and the angle ABT may be observed; and if the angle ABT
is, as shown in Fig. 1, a right angle, then the sin ABT becomes unity. It remains,
therefore, to find the angle ATB in order to determine the distance AT; so that it
becomes necessary to provide a simple and rapid means of at once determining what
the angle ATB is. To this end, I connect the arcs E, F, of conducting material, in a
Wheatstone bridge, the four members of which are shown respectively at a, b, c, d.
In this bridge I connect a galvanometer g in the usual way; and also the battery /;
the terminals of the battery wire being connected to the telescopes at their pivot
points A, B; so that the circuit proceeds through the telescopes to the arcs, and
then at the arc F divides through the wires b, d, and at the arc E divides through
the wires a, c.
It will be plain that when the two telescopes C and D stand at
right angles to the base line, and hence parallel one to the other, the bridge will
balance, and the galvanometer will show no deflection. The lines of sight of the
two telescopes then being parallel, the galvanometer will then indicate infinite
range; and of course, this will be true no matter where the telescopes may be on
their respective arcs, so long as their lines of sight are relatively parallel. But
if one telescope be moved out of parallelism with the other, as for example, the
telescope C moved to the position C³, then clearly the bridge will be thrown out of
balance, and the galvanometer will be deflected. It will also be clear that the extent
of deflection of the galvanometer will depend upon the length of are included
between the two positions of the telescope, C, C¹, and will be greater as that arc

page_text: increases. So that with a battery of constant electro-motive force, it becomes
possible to determine the extent of imovement of the telescope C by simply observing
the indication of the galvanometer. It will of course be obvious, that as the angle
between the positions C and of the telescope increases, the length of the line AT
will constantly decrease, while the deflection of the galvanometer will constantly
increase: So that the galvanometer indicates ranges starting from infinity when
the galvanometer shows no deflection; small ranges being indicated by large
deflections of the galvanometer, and vice versa. As a matter of convenience, I prefer
to employ for this purpose a galvanometer so constructed that the deflections of the
index will be proportional to the difference of potential at the terminals. Instruments
of this type are well known in the art, and need no special description here. It
will be clear that by the foregoing means, I have reduced the operation of finding
the range to a very easy and rapid process, and at the same time, greatly simplified
the apparatus.
Observers stationed at the two telescopes C and D align them
with the distant object, when a third observer instantly reads the range from the
galvanometer which is provided with a scale suitably marked in linear units, such
as yards.
If, however, the angle ABT is not a right angle, then the factor sin ABT must be
taken into consideration in solving the formula AT =
AB
sin ATB
× sin ABT. Or in
other words, the observer at the galvanometer may simply multiply the range
indication by the sin ABT numerically expressed, in order to reduce the indicated
range to the true range. The angle ABT is observed directly on the arc F.
To
In the foregoing demonstration, it is assumed that the resistance in the circuit
remains constant. Or in other words, remains the same as it is when the two
telescopes are parallel to one another, and stand in the positions C, D, touching the
middle parts of their arcs. But as a matter of fact, this resistance does not remain
the same when the telescopes move to positions nearer the extremities of the arcs.
illustrate, if the resistance of the circuit is a certain amount with the telescopes in the
position C, D, it will be less when the telescopes are turned in the position C¹ D¹.
Now the variation of resistance due to this change of position will affect the total
resistance in circuit to an extent depending upon its ratio to the resistance of the
whole circuit. And consequently, if that ratio be made very small, as it easily may
be by simply introducing a high resistance in the battery loop at i between the
points A and B, then inasmuch as the variation in resistance due to change in position
of the telescopes may thus be rendered inappreciable, the total resistance of the
circuit may be taken as constant; so that despite the fact that the angle ABT differs
from a right angle, the deflection of the galvanometer as before, will remain
practically constant for any given angle ATB.
I am aware that if the high resistance before mentioned be not put in the battery
loop, then the decrease in resistance due to change in position of the telescopes from
the middle point of their arcs toward the extremities of the arcs may bear a con-
siderable ratio to the resistance of the whole circuit. And as this decrease in
resistance will be attended by corresponding increase in current strength, it follows
that proportionately greater deflections of the galvanometer will follow for any given
angle ATB. So that consequently, the ranges indicated by the galvanometer will be
less than those which would have been shown had the high resistance been put in the
battery loop. And if the resistance of the battery loop between A and B is extremely
small with relation to the rest of the circuit, the decreasing resistance of the whole
circuit due to change in position of the telescopes, may become very large; and that
this result may be intensified if the members a, b, c, d, connecting the arcs, are
connected to those arcs at points less than 90 degrees from the middle points of those
ares. If, for instance, these wires were connected to the ares at points 81 degrees
removed from the middle points, and if the resistance in the battery loop were
one-tenth of the arc of 81 degrees, then when both telescopes were moved to positions
60 degrees from the middle points, the resistance of the whole circuit would then be

page_text: only about half of what it was when the telescopes were at the middle parts or
the positions C, D. Consequently, for any given relative angular displacement of
the telescopes occurring 60 degrees away from the middle points of the arcs, the
corresponding deflection of the galvanometer would be about twice as great as if
the same relative angular displacement occurred when the telescopes were near the
middle points of the arcs; so that the range indicated in the latter case, would be
about half as great as in the former. But it will be observed that if the telescope D,
for instance, were 60 degrees removed from the central position, the angle ABT
would be 150 degrees, and then its sine would be one-half. So that the range..
indication for any given angle ATB, would be only one-half of what it would be
with the same angle ATB when the telescope at D¹ is in its middle position. In
other words, the fact of the decreased resistance caused in the circuit as the telescopes
move away from the middle position, tends to automatically make the very correction
for the sine of ABT which ought to be introduced because the telescope no longer
stands at 90 degrees to the base line; and this I find to be the actual occurrence in
practice. It is not asserted here that this correction is mathematically exact. I am
merely pointing out the phenomena which occur when the telescopes are pointing at
objects at small angles to the base line, and showing how the reading of the galvano-
meter can be depended upon to show at once the approximately true range with
the corrections above indicated.
In the above the resistance of the galvanometer has been neglected, and it has been
assumed that the E.M.F. and internal resistance of the battery, and the resistance of
the various contacts, remain constant. While this is not theoretically true, I have
found that by using storage batteries and by making the contacts carefully, no
appreciable error is introduced. Careful experiments with my range finder at sea,
show that the errors of observation are much greater than those of the instru-
ment itself, and for this work, where precise measurements are not required, but
quick reading is essential, the errors above mentioned need not be considered. In the
case where the resistance of the circuit is made small, it will not be necessary under
ordinary conditions, to make both the correction for sine ABT and the correction for
the increase of current due to the object being at an angle to the base line; but since
these two connections are in the opposite sense, and can be made approximately equal,
it is sufficiently accurate for practical purposes to take the uncorrected reading of the
galvanometer.
It has been assumed that the angle ATB is so small that its sine may be considered
as proportional to the angle, and also that the deflection of the galvanometer for any
angle ATB is approximately proportional to the current strength in the said circuit,
and the fall in potential per unit length in the arcs, which I find in practice to be
the case.
It is obvious that two or more galvanometers may be used being placed either in
series or in multiple arc with each other.
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. Finding the range of a distant object by first determining a fractional portion
of a conducting body bearing in length a ratio to the angle included between two
lines of sight directed upon a distant object, and simultaneously causing a disturbance
in an electrical balance including said body in its circuit, proportional to the
resistance of said fractional portion; and second, observing the difference in potential
in the said circuit due to said disturbance.
2. Finding the range of a distant object by first determining a fractional portion
of a conducting body bearing in length a ratio to the angle included between two
lines of sight directed upon a distant object, and simultaneously causing a disturbance
in an electrical balance including said body in its circuit, proportional to the resistance
of said fractional portion; and second, reading directly from the suitably marked
.

page_text: scale of an electrical measuring instrument constructed to measure the amount of said
disturbance, the range corresponding to said angle between the lines of sight.
3. The combination and arrangement in a range or position finder, of two bodies of
conducting material similarly disposed with reference to a predetermined base line,.
alidade arms establishing and maintaining contact with said bodies, and an electrical
measuring instrument constructed to measure differences in electrical potential; the
aforesaid parts being connected in electrical circuit substantially as set forth; the said
measuring instrument being provided with an index and a scale marked in units
of linear measure.
Dated this 28th day of August 1890
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, Agents for the Applicant.