An Improved Float for Determining the Absolute Weight and Specific Gravity of Solid and Fluid Bodies.
Year: 1894

page_text: Date of Application, 19th Mar., 1894-Accepted, 12th Jan., 1895
COMPLETE DI DUITICATION.
An Improved Float for Determining the Absolute Weight and
Specific Gravity of Solid and Fluid Bodies.
THEODOR LOHNSTEIN, of No. 20 II Kaiser Wilhelmstrasse, Berlin, in the
German Empire, Doctor of Philosophy, and Medical Undergraduate, 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 instruments known as areometers hitherto employed in determining the
specific gravity of liquid and solid bodies, and based upon the application of the
Archimedean hydrostatic principle have all exhibited a very marked tendency to
become unreliable under the influence of capillarity. As is well known, according
to the nature of the liquid and solid bodies which are brought into contact with
each other (including the body of the areometer itself,) there forms upon the
spindle of the instrument, as a result of capillarity, a concave depression or convex
swelling, which is apt very considerably to affect the depth of immersion of the
spindle. Hence the application of the Archimedean principle only, in the con-
struction of the scale of the instrument, must necessarily become somewhat
misleading. The influence of capillarity, may indeed, be ascertained by
computation; but the corrections thus arrived at depend upon two physical
quantities, in addition to the diameter of the areometer spindle; namely on the
superficial pressure of the liquid, and on the angle which the surface of the liquid
forms with the surface of the casing of the areometer spindle, in the line of contact.
The first of these quantities, provided the liquid is absolutely pure both in its body
and surface, is a constant one, though it is apt to vary very considerably whenever
there is the slightest impurity in the liquid. But more important variations still
are liable to occur in the case of the second quantity viz. the so-called "marginal
angle" which is dependent upon the absolute purity of two surfaces. If these facts
be true in the case of one particular liquid it follows that in establishing a scale
for an instrument intended for testing liquids of every conceivable kind or
description, the influence of capillarity cannot be calculated with anything like
reliable accuracy.
In the case of those areometers the readings of which vary within a wide range,
say from 0.7 to 1-0 or from 10, to 2.0 capillarity, as a source of error, does not call
for very serious consideration, since so long as the readings fluctuate between
ordinary dimensions, variations arising from this source would generally occupy the
second place after the decimal, at the outside; but in instruments in which, the
range being narrower, greater precision is necessarily the object, the inconvenience
due to capillarity is apt to manifest itself in a very objectionable manner; in
addition to which correct reading is interfered with by optical peculiarities.
These drawbacks are entirely obviated by the improved areometer constructed in
accordance with this invention, which comprises a novel arrangement whereby the
usual swelling produced by capillarity is entirely disposed of; thus enabling the
Archimedean principle to be strictly carried out.
As shown in Figure 1 of the accompanying drawings, the instrument consists of
a hollow body A, made of glass, and terminating in an open cylindrical extension.
This extension, being cut off exactly at right angles to its axis, has an edge B
which, from optical considerations, is cut or ground in such a manner that facets of
any kind are entirely avoided.
.

page_text: Within the cylindrical end piece a nut C. made of metal or hard rubber, is
cemented for facilitating the attachment of the wire frame DEF carrying the
weighing basin or dish G. The said frame consists of a vertical rod D, a horizontal
rod E and two more vertical rods F to, which the dish or basin G is secured in any
suitable manner. To the dish is attached the indicator H which indicates the
vertical or perpendicular position of the floating body A. The rods or wires F are
sufficiently long to permit the float A to be readily inserted into the vertical
cylinder, which receives the liquid, without necessitating the.disconnection of the
screw joint at C. The cylinder I rests upon a stand or support K of suitable
construction. Any too extensive displacement or undue sinking of the float may
be prevented by very simple arrangements. The weight of the unweighted float is
so balanced that it will float in a vertical position, in a liquid of 0.7000 specific
gravity and at a temperature of 15° Centigrade, and the liquid will reach to the
extreme edge of the top piece B, while any capillary deflection of the surface of the
liquid totally disappears throughout the marginal line.
When however the unweighted float is placed in a liquid of greater specific
gravity, a proportional part of the float only will be immersed in the liquid, or
situated below its surface. By gradually weighting the float, a condition of things
will be first brought about in which the liquid, while moistening the whole surface
of the float, will with its free surface o' o' touch the external sharp edge of the top
piece B; and by continuing the weighting operation the capillary deformation of the
surface o o¹ may be caused to cease entirely, so that the surface of the liquid forms
an exactly horizontal plane or level surrounding the float.
Under these circumstances the Archimedean principle is strictly applicable.
Errors resulting from the influence of capillarity are entirely precluded under the
circumstances above mentioned. It is however not always possible to attain this
position of the float, which is dependent upon a certain distribution of the masses
in the interior of the said float, it being necessary that the geometrical centre of
gravity of the volume of the float and the actual centre of gravity of the whole
apparatus fall in a straight line perpendicular to the surface of the fluid. By
applying sliding poises and counter poises to the scale of the apparatus and suitably
arranging the same, the position of sharp edge of the float as above stated could
easily be attained in each and every case. Such complication of the apparatus is
however unnecessary.
Innumerable other cases may be cited in which the
Archimedean principle is applied without correction. Suppose the weighting
process above described is carried out with a float not quite exactly balanced, the
curve of the upper surface will not be exactly the same at every point of the
circumference of the upper end of the float, owing to the latter not lying in
the horizontal plane. Thus the curve will disappear at one point of the edge of the
said float, concave curves being formed at the remaining part of the circumference.
If the apparatus is now more heavily weighted, convex.curves of the upper liquid
surface will be formed at the first mentioned point, whilst at a point diametrically
opposite the same, the concavity of the curve remains.
It is easy in this manner to attain a position in which the convex curve extends
round one half and the concave curve around the other half of the circumference of
the upper end of the float (Fig. 2). With this condition of things as may be easily
theoretically proved, if it were not self evident, the weight of the float is equal to
the weight of a volume of the liquid equal to the volume of the float. The
observer's eye will require but very little practice to be able to detect this condition
of things, where it exists, by simply observing the reflections of the surface of the
liquid.
The precision of the improved areometer is mainly due to the relation existing
between the cylindrical end piece and the total volume of the float. Supposing,
for example, that the former is 6 millimeters and the latter 30 cubic centimetres,
and assuming that an average deviation of the upper marginal line, from the level
surface of the liquid amounting to only 001 millimeters, is observable (which
result can be obtained with very little practice) this instrument will enable the

page_text: specific gravity of the given liquid to be ascertained within the proportions of
1 to 30000, provided that the capillary pressure is as high as it generally is in the
case of perfectly pure water. I am therefore justified in stating that this
instrument, if constructed as stated, in regard to its size, will enable the specific
gravity of liquids to be accurately determined, at least as far as the four first
figures after the decimal.
For the weighting of the float there is provided, in addition to the arrangement
herein described, a set of ordinary weights which need not here be further dwelt
on, and which are so arranged that they enable the specific gravity of the liquid
which is being tested to be read off from them direct.
To obtain precise readings, the temperature should also be taken into account.
But as this point is regulated by the well known rules of physical science, it need
not be further considered here. And, likewise, it will be sufficient merely to
mention in a cursory way that for special purposes, such as aërometry, instruments
based upon the same principle may readily be constructed in a simplified form. It
will also be easily understood that the principle of this invention may be extended
also to the calculation of the specific gravity of solid bodies, if required, it being
only necessary for this purpose to use the rod of a Nicholson areometer in lieu of
the edge or "mark" here described, and to provide it with a sharp edge.
Another possible application of the same principle which deserves to be specially
mentioned, is the construction of hydrostatic substitution scales. Supposing for
example that the present areometer is first weighted with a full set of weights, and
next, after the removal of part of the weights, with any other object; it will be
obvious that the weights removed will indicate or represent the weight of that
object. By direct observation the sensitiveness of a scale such as this may be
brought down to one milligramme, while greater weights may be easily brought
under the control of the instrument by increasing the bulk, and therefore the
weight, of the float. Temperature will influence this weighing process only to the
extent to which the temperature of the water is apt to change during the time
occupied in the test, and which does not exceed a few minutes. This source of
error may of course be provided against by the aid of a suitable table of reference.
A great advantage of this scale is its very moderate cost. By foregoing direct
observation on the surface of the liquid, and employing suitable auxiliary devices
enabling the results of the test to be read off by means of a small microscope
assuming the maximum weight. to be about 500 grammes, the precision of the
instrument can be brought down to 0.1 millimetre. This, of course will make the
instrument a little more expensive, but its cost will still be far below that of any of
the ordinary instruments hitherto employed for like purposes.
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 [
claim is :-
A float serving for determining the absolute weight and specific gravity of
bodies, mainly consisting of a sharp edge terminating it at the top and serving as a
mark up to which the said float sinks down in the liquid which carries it, under
the action of a suitable weight, with the result that the liquid along the periphery
either presents no curve, or forms both concave and convex menisci equal in
extent.
MARKS & CLERK,
18, Southampton Buildings, London, W.C., Agents.