Pikaõlalised analüütilised kaalud Tartu Ülikooli muuseumis

Abstract

Long-beam Precision Balances at the University
of Tartu Museum
Tullio Ilomets
University of Tartu Museum

There are a number of historical long-beam precision balances in the
University of Tartu Museum collection, most probably dating back to the
first half of the 19th century; this article introduces two of those balances
more closely. According to oral tradition, the first of the balances belonged
to Carl Ernst Heinrich Apollon Schmidt (1822–1894), a professor of
chemistry at the University of Tartu in 1847–1892, and the balance was
preserved in the department of chemistry chair of inorganic chemistry.
The balance was handed over to the museum in 1982. The second
large analytical balance, a so-called one-kilogram scale was made in
the workshop of the famous Berlin balance craftsman Johann August
Daniel Oertling (1803–1866). The balance bears the inscription: Aug.
Oertling in Berlin № 1329. The balance was handed over to the museum
by the chair of inorganic chemistry on 11 June 1993.
The balance of Carl Schmidt can be disassembled into five parts
that are packed into the drawers of a mahogany balance case, each
part to its proper place. The balance is made of light brass. The measurements
of the balance case are 51.5 x 13.5 x 8.5 cm. The drawer of
the case can be locked. On top of the case, there is a brass plate with
the measurements 3.0 x 3.0 x 0.6 cm in the centre of which there is a
threaded opening, where the column is connected. The front edge and
edges of the balance case have metal feet the height of which can be
adjusted. The main part of the column supporting the balance beam
is a 1.9 cm-in-diameter solid brass cylinder, while the eccentric and
scales are situated just below it. There is a console attached to the top
part of the column; on top of that, there are two round agate plates,
which support the knife edge of the beam’s prism. The full length of
the column is 28.5 cm. The moving part of the column consists of two
cylinders that are connected with three rods, are positioned around
the column and can be moved up and down. A frame with two notches
surrounding the console and a forked arrestment device for the beam
positioned perpendicularly with the frame are attached to the upper
cylinder. The lower edge of the lower cylinder touches the eccentric.
If one rotates the eccentric, the frame moves up and positions itself
in the notches above the prisms’ ends, the forked arrestment device
fixes both sides of the beam in the grooves at the ends of the forks.
This system of arresting the balance beam was first used by August
Oertling. The arresting system of Carl Schmidt’s balance is very similar
to an Oertling-type balance described in older literature. This
except for the fact that in case of Oerlting’s balance, the arrestment
system of the balance is lifted by the eccentric with the aid of a rod
within the column. Carl Schmidt’s precision balance has a rhombic
structural beam with two vertical crossbars. The beam is graduated
on one side for the manual positioning of the rider. The length of the
beam is 42 cm. In the middle of the beam on its upper edge there is a
device that allows regulating the centre of gravity of the beam; on the
lower edge there is a downward pointed needle. On both ends of the
balance beam there are the so-called swan necks that are bent upwards
into curves and have two regulating screws; their lower ends
are circle-shaped and the suspension threads of the balance bowls
are attached to these. Oertling’s balance also has these swan necks.
The balance has no notations regarding its manufacturer, as well
as the time and place it was made. Neither does it have a university
inventory number. We may presume that the balance is an earlier
version of an Oertling-type balance and it belonged to Carl Schmidt
personally. Judging by construction, the balance was made in the
first half of the 19th century.
Oertling’s large analytical precision balance is made of brass and
preserved in a mahogany case, which has glazed doors on the sides and
glazed wooden frames that can be lifted to the required height on the
front and back sides. There are two brass feet with adjustable height
under the balance at the corners on the front side and a single brass foot
in the centre of the back side. The balance case can be locked. There are
dampers that hinder the oscillation of the balance bowls in the bottom
of the balance case. Front centre, there is a round fluid level for levelling
the balance. Under the balance bowls, there is a round hole in the bottom
of the balance case, and it has a brass cover that can be slid aside.
A device for mounting the reiter balance beam is attached to the ceiling
and right-hand inner wall of the case. The measurements of the balance
case are 80.0 x 54.0 x 24.0 cm. The height of the lower part of the case
is 6.0 cm. The balance column is attached to a brass plate measuring
16.0 x 8.3 x 0.7 cm at the bottom of the case. At the point where the lifting
mechanism side forks come out of the column, the diameter of the
column is 4.0 cm, upwards, it is 3.0 cm. The total length of the column is
27.7 cm. A plate measuring 4.0 cm x 4.0 cm x 0.9 cm has been attached
on top of the upper plate of the column; on top of that, there are two
parallel-positioned 5.8 cm high bearers that are 1.2 cm apart – these
are meant for supporting the round agate plates. A scales mounting
frame with a rhombic structure attached with six vertical crossbars is
attached to the back of the column; its length together with the scales
is 61.5 cm. The moving part of the balance column is made up of the
beam’s arresting frame that is 67.0 cm long and consists of two halves
2.8 cm apart from each other. Both sides of the frame have a rhombic
structure as well and they have six vertical crossbars. The central part
of the frame, which is 5.4 cm high, is attached to the plate at the bottom
on both sides; the plate features holes that fit the agate plate bearers
exactly. The plate is connected to the tip of the rod that moves up and
down inside the column. A triangular notch is aligned with the prism
at the extension of the upper side of the frame’s central part. The two
sides of the arrestment frame have been connected with cross strips,
while external rods that are 30.0 cm long, reach out from the openings
in the lower part of the column, and are connected to the rod inside the
column, are attached to the strips. On the ends of the arrestment frame,
there are triangular notches above the balance beam’s end prisms on
the upper edge. Upon arrestment, the frame lifts the middle prism of the
beam from the agate plates into the notches. At the ends of the beam,
the frame lifts the forked suspension frame of the balance bowl together
with the agate plates from the prisms into the notches. The upwards
and downwards movement amplitude of the frame is 1.0 cm. The balance
can be arrested with a disc on top of the balance case, which is connected
to a lifting mechanism inside the column via the axis. Oertling’s
large scale has a rhombic beam with six vertical crossbars; its length
is 55.0 cm, its width at the point of the supporting prism is 5.0 cm and
thickness 0.9 cm. In the centre of the beam on its upper edge above the
supporting prism there is a device for regulating the centre of gravity of
the balance beam. The beam has no pointer. The ends of the beam are
forked. The upper forks have points that reach the scales. The beam is
balanced when the points are on 0 noted on the scales. The lower forks of
the beam ends have prisms that carry the balance bowls. The front part
of the beam’s upper edge on both sides of the prism is graduated. There
are two graduations: upper and lower. The lower starts from the middle
prism and runs to 10. The upper graduation starts from the middle of
one side of the beam with 0 and runs to 10 on both sides of the 0, while
each of the units is divided into five parts in their turn. The diameter of
a balance bowl is 10.0 cm. Under the balance bowl in the middle, there
is a loop, which can be used to attach a fine wire or a hook and thread
that come from the opening under the balance bowl. This allows to use
the balance as a hydrostatic balance.
As there are no points of comparison, the time when August
Oertling’s long-beam precision balance No 1329 was made is unclear
for the time being. If we are to presume that the balance was made
during Oertling’s lifetime, it could be dated to the first half of the 19th
century; the number most probably denotes the serial number of the
manufactured devices.
The article provides a short overview of the history of long-beam
precision scales, mainly on the basis of the researches of Hans R.
Jenemann on this subject. Jenemann defines the age of the long-beam
precision balances with the years 1775–1875. The article also briefly
discusses the achievements of the preceding age in perfecting assay
balances. The author also introduces the basics of balance theory in
connection to the possibilities of increasing the sensitivity of balances.