AN ISOMETER OR DYNAMIC SECTOR
Year: 1883

page_text: LETTERS PATENT to Herbert John Allison, of the firm of Allison Brothers,
Patent Agents, Southampton Buildings, London, for an Invention of "AN
ISOMETER OR DYNAMIC SECTOR," Communicated to him from abroad by Henry
Glover, of Brooklyn, New York, United States of America,
COMPLETE SPECIFICATION filed by the said Herbert John Allison at the
Office of the Commissioners of Patents on the 14th August 1883.
com-
HERBERT JOHN ALLISON, of the firm of Allison Brothers, Patent Agents,
Southampton Buildings, London, "AN ISOMETER OR DYNAMIC SECTOR,"
municated to me from abroad by Henry Glover, of Brooklyn, New York, United
States of America.
My invention which I term an Isometer or Dynamic Sector, relates to an
improvement in that class of philosophical instruments which are termed boussoles
and is applicable to all such apparatuses in their various forms, where a pivoted
magnet needle is employed to graphically indicate the magnetic meridian which
passes through the centre of the point of observation and which serves as the basis
for further manipulations, such as locating the true meridian as well as its
respective magnetic meridian to properly define the latitude of any given point, to
observe the exact amount of local disturbances which effect the magnet needle, as
on board of iron ships, and for other purposes hereinafter more fully described.
As is well known the magnetic meridian differs from the true meridian, by
assuming daily a different position increasing or decreasing either to the east or
west of the true meridian and producing constantly different bearings at the various
points of observations which necessitates a careful and protracted correction by
calculating such difference and bringing the result to bear on the different
observations.
It is furthermore well understood that local attractions of various known and
hidden causes will seriously interfere with arriving at any true and efficient result,
many precautions being taken to secure to the mariners compass, especially on iron

page_text: ships, a uniform action to become unaffected by any local disturbances and solely
subject to the deflection of the magnetic meridian, have not proven sufficient to
secure at all times and at all points and at each change of the ship's course a
mathematically correct base of errors.
Among other purposes, my invention is designed to remove this objectionable
feature of calculating for and with the errors, and is so arranged and proportioned
that by simply noting the deflection of the needle on a vernier scale after the
instrument has been adjusted, the exact amount of errors caused by local disturbances,
change of the ship's course or by other causes will be obtained in points or degrees
and minutes.
In the accompanying drawings.
Fig. 1 illustrates in plan view the application of my invention to an ordinary
boussole or compass.
Fig. 2 illustrates the same in a longitudinal vertical section, and
Fig. 3 shows an ordinary transit or theodolite provided with my apparatus.
In Figure 1, A is the index lever, fulcrumed radially to the pivot P of the free
needle F and carrying on its outwardly extending extremity the vernier plate B.
Rigidly attached to the instrument and extending radially outwardly is the
sector S with vernier scale C. The latter is divided into thirty-two equal points
to either side from the centre or Zero point, while the vernier plate B is divided
into eleven equal parts bearing a proportion of eleven to ten of the vernier
scale C. The object of such proportions will be hereafter more fully explained.
In Figure 2, G. illustrates a fixed needle or magnetic bar solidly attached to the
-under-side of the main plate and parallel to the centre-line of sector. S, and is set
in a plane formed by the Zero point on the vernier scale and the pivot P of the
free needle. An index point K is located in the centre line upon the index lever A,
and slightly projecting above and beyond the north end of the free needle F,
whenever the latter is brought to point in this direction.
To the standard L may be attached a magnifying glass to facilitate the reading
on the scales.-
The fixed needle G will exert its influence upon the free needle and cause the
latter to assume a nearly like direction whenever the instrument has been partly
revolved in azimuth. The earth's magnetic meridian passing through the centre
of this instrument will also exert its influence upon the froe needle F, and cause
the same to deflect to the right or left of the index K to such an extent as the
point of observation and its position and bearing to the true meridian warrants.
In case now the index lever A is revolved to such an extent that its index K will
coincide with the north end of the free needle F, then the position of the vernier
plate B on the scale C will indicate in points the difference between the true and
magnetic meridian, that is; assuming that the fixed needle represents the true
meridian then the difference between the magnetic and true meridian is shown by
the arc of deflection between both, viz; the free statical resisting needle, being
submitted to two opposing forces, will assume according to the well known law of
forces a diagonal direction to the resistance offered.
It is evident that when the instrument is revolved on its own axis in simple
azimuth from the true meridian, the free needle will deflect from the zero or
meridian line of the instrument to a point, whose arc equal in amount in degrees
to the latitude of the place of observation and no more the same amount
considered as intensity is a measure of the earth's total force for that latitude.
But when the instrument is on the true meridian of the place of observation then
there is a perfect coincidence of the free needle and the index point, and a
parallelism of the free and fixed needle, and any divergence whatever of the
instrument from the true meridian, either by rotating the instrument in azimuth
or by local disturbances, will cause a corresponding deflection of the needle,
consequently at any time or place independent of solar observation or local
magnetic disturbances a true meridian point can be determined by simply revolving
the instrument in azimuth until the free needle coincides with the zero or index

page_text: point, Any local disturbance which might operate in part to deflect the iree
needle can be easily ascertained by placing the instrument in azimuth East and
West and then rotating the index lever as many points as the plan of observation
warrants. Should the north end of the free needle not coincide with the index K
on the index lever, then the amount of points which it is necessary to further
rotate the latter will indicate the erroneous deflection caused by local disturbances.
Such points reduced to degrees, minutes and seconds will serve as a base of errors
in the various manipulations for which the apparatus is employed. As per
illustration: The instrument being situated at a point in New York City, and
placed in azimuth to a point coinciding with its lateral bearing viz 40° 40'. The
deflection of the needle at this point being equal to 3. 6 points and the index being
set to this scale, a deflection of the free needle is still observed, which measured on
the scale, after the index is brought to a coincidence with the end of the needle
and now indicating 6. 8-shows 3. 2 points caused by local disturbances at this
point.
In Figure 3 of drawings a practical application to a surveying transit is
illustrated in which M represents the graduated scale of the instrument serving as
simple azimuth in the operation of my apparatus and N any well known device
for adjusting the same. F is the free needle, K the index point, A the indox lever,
O the vernier scale and B its plate.
As all surveys for measuring distances to determine local and geographical
boundaries etc, depend upon the accuracy of finding a true meridian, and as
experience has shown that in many geographical localities the local disturbances of
the magnetic needle are so variable that it is with existing means very difficult to
obtain perfectly satisfactory results, free from instrumental errors, hence my
invention as applied to this class of apparatuses in correcting its own error and
independent of celestial observation will give perfectly true results.
In applying my invention to nautical instruments its prime object will be, that
of determining the variation of the true from the magnetic meridian, to correct
the error caused by local disturbances so difficult to combat in iron ships.
Although this error can be detected by celestial observations and other similar
ingenious devices, still certain atmospheric, conditions such as foggy and cloudy
weather, will at times prevent the usage of the same.
In great many instances the ship's cargo and equipment will exert its magnetic
influence upon the mariner's compass and will cause the magnetic needle to diverge
from its normal direction to such an extent as to render the same entirely useless
for the purpose of navigation. Especially would this be the case when the navigator
should leave port and almost complete his voyage with constant thick and cloudy
weather, with the ship's compass as his sole guide, being influenced by the magnetic
condition of the ship's cargo.
From the fact that the error varies in amount by each change of the ship's
course and every change from a vertical position by the ship laying over, the error
will increase or decrease with the larboard or starboard angle of the ship, which
makes a true solution of this problem of error very difficult.
It is apparent that an instrument that will indicate the error at any time so
that the true course can be estimated for any distance will be especially
advantageous for this purpose.
The construction of the scale on the sector with its appending vernier is as
follows. The division of O to thirty-two corresponds with the thirty-two points
of the azimuth scale, in such a manner that the vernier O to eleven represents the
thirty-second part of the circle, this part is divided into eleven parts for the vernier
and into ten equal parts for the scale. Thus one point of arc on the sector equals
the 32 part of the circle, which give the value of 11° 15' to ench point, that is the
force that drives the free needle, while controlled by the fixed or secondary
needle a distance of one point, as observed on the sector of my Isometer, will drive
a free needle of an ordinary compass 11° 15'.
Therefore by dividing the vernier scale at the proportion of eleven to ten the

page_text: result will produce a tenth part of a point equal to 1. 75° instead of the usual
sub-division of degrees in minutes and seconds.
The relative length of the free swinging needle to the fixed needle is proportioned
as eighteen is to sixteen which corresponds to the difference between the equatorial
and magnetic planes, viz: assuming the fixed magnetic needle to represent the
true meridian of the instrument indicative of the earth's magnetic plane, while
the free swinging needle represents the magnetic meridian relative to the earth's
equatorial plane then there is a difference of two points actually existing between
the centres of both planes of be transferred to the length of the respective needles.
Assuming the Isometer to be located at a point on the earth's equator between
the centres of the equatorial and magnetic planes then the north and south ends
of both needles will indicate the radii of the earth's and magnetic equators. If
now the radius of the earth's equator is divided into nine equal points or parts
and one part thereof considered as unit and marked off on the radius of the
magnetic equator then the latter will be equal to a length of eight parts, the
difference of one part or point on each radius of the fixed needle or two points
for the entire length of the same arising as aforesaid from the actually existing
distance of two points between the two centres of both planes.
Therefore by giving length of eighteen parts to the free needle (proportionate
to a radius of the earth's equator) and of sixteen parts to the fixed needle
(proportionate to the magnetic equator) the same will bear relative proportions to
the equatorial and magnetic planes.
But I do not confine myself solely to these proportional lengths of both needles,..
-as in practice two needles made of equal length or of any other proportional
length will not show any visible difference in an instrument with a sector of about
five inches such as is best suited for ordinary use.
What I claim as my invention is:
1 The combination with the fixed needle and a vernier scale each of whoso
units is of the circle upon which it is formed, and is sub-divided into-tenths, of
a free swing needle mounted on a support that is provided with a vernier
subdivided into elevenths of the unit for the vernier scale.
2nd The combination substantially as described, of a fixed magnetic needle, a free
swinging needle, mounted over or under the same and upon a support that swings
on a centre coinciding with the fixed needle, and means for measuring the relative
angle of the fixed and free needle after adjustment.
3rd The combination of a fixed magnetic bar or needle and a free swinging
needle mounted on a support adjustable in the azimuth plane of nautical and
surveying instruments, as and for the purpose described.
4th The combination substantially as described, of a fixed needle and a free
pivoted needle proportioned as described.
In witness whereof, I, the said Herbert Jobn Allison, have hereunto set my
hand and seal, this 14th day of August in the year of our Lord one thousand
eight hundred and eighty-three.