STANDARD - Survey Instrument Antique Center!

99
Magnetic Needle of Edge-bar Form
The needle shown in the cross section of our Transit compass also in the top view
on opposite page, represents the form adopted and preferred by us for all of our com-
pass instruments, because it has its greatest dimensions in the vertical direction hence
;
its name. At the ends, where it is read, it is quite thin, but increases in thickness
symmetrically towards the central part to give it the rigidity necessary to retain the true
longitudinal shape and yet be very light of weight to minimize the dulling of the pivot
on which it swings. The point of suspension in the steel cap* and the two ends of the
needle are in a straight line, thereby forming the geometric axis.
The advantage derived from the edge-bar form, therefore, is that its magnetic
axis must be contained in the geometric axis of the needle, whence it
follows that there is no index error at its reading ends.
This cannot be claimed for a needle of the flat, oblong type, since its magnetic
axis may follow the grain imparted to it in rolling the steel ingot. In such a needle
the index error may be negligible or amount to a great deal, according to size and
shape. Hollow, cylindrical, and elliptically shaped needles are not exempt from this
error, and as their ends require brass extensions for reading the graduation in a surveyor's
compass, the effective length of the needle is not only shortened, but the extensions
in themselves may become an additional source of index error. As will be seen
in the cross section, the center of gravity in the edge-bar form is very much below the
point of suspension, which, together with the increasing weight of the needle toward
its central part, makes it less sensitive to dip. The quivering of a needle so constructed
is not annoying, since the center of its quivering motion is in line through its two extreme
points, which are, therefore, stationary. Permanent magnet steel is used only
in its construction, which, when properly hardened, will, at all times, retain sufficient
magnetism to give the needle direction when resting on a sharp point. It is the dulling
of the latter t which is commonly at fault when a needle does not settle repeatedly in
the same place not loss of magnetism as is generally supposed and most of this
difficulty may be obviated by carefully raising or lowering the needle and not allowing
the needle to play when shouldering the instrument. To be light the steel cap is mounted
in an aluminum cell.
* It is our belief that no advantage whatever is gained by suspending the needle on jewel
bearings, but that a properly tempered steel bearing, resting on a steel pivot of a different temper,
is superior to the jewel mounting. We find that in the instruments sent us for repair the jewels
are almost always grooved by the wear of the pivot so that it is difficult, or impossible, to repair
them satisfactorily. With steel bearings no such difficulty is encountered. As to danger of rusting,
it is certainly no greater for the bearing than for the pivot, and the instruments sent in for
repair do not often show signs of either bearing or pivot rusting.
t In order to partially remedy the wearing on the point we sometimes, upon special request, furnish a
needle of twisted form which can be made very Tight and be quite stiff in proportion to its weight. It is not
of a strictly -scientific shape but answers the requirements of a Surveyor's Compass. , ; The needle consists of a
flat bar of ye 5
thin permanent magnet steel. At the central portion of; the ,bar, the blade is horizontal,
while at the ends it is vertical as in the edge-bar form of needle. Between the center and the ends there are
several twists in the bar. The points of the needle and the point of suspension are all in the same straight
line in this new needle, as in the old. The center of gravity is placed as far below the point of suspension as
the necessarily limited depth of the Transit compass permits, but owing to its extreme lightness the center of
gravity should be carried much farther bilow than the old to counteract the dip to the same extent. In consequence
tlie needle of twisted form is more easily affected by change in latitude and its
application is, therefore, limited, to use within latitudes differing not more than a few degrees.
Caution.
The magnetic needle must always be lowered very gently on its pivot. The
knob marked "Lifter" (see page 98) raises and lowers the needle. If the needle is
lowered abruptly upon the pivot, its fine point may be dulled the first time, and the
needle will then work sluggishly and not settle twice in the same place.
To use the Variation Plate, insert an adjusting pin into the capstan-headed nut
beneath the "Lifter," and by turning either way the desired declination may be set
for East or West.