AN IMPROVED CONSTRUCTION OF BAROMETER
Year: 1863

page_text: LETTERS PATENT to William Haslett Mitchel, of Hampstead, in the
County of Middlesex, Gentleman, for the Invention of "AN IMPROVED
CONSTRUCTION OF BAROMETER."
Sealed the 2nd October 1863, and dated the 8th April 1868.
PROVISIONAL SPECIFICATION left by the said William Haslett Mitchel
at the Office of the Commissioners of Patents, with his Petition, on the
8th April 1863.
I, WILLIAM HASLETT MITCHEL, of Hampstead, in the County of Middlesex,
Gentleman, do hereby declare the nature of the said Invention for "AN
IMPROVED CONSTRUCTION OF BAROMETER," to be as follows:-
The chief object of this Invention is to produce cheap and portable baro-
meters, also when desirable to give them a more extended scale or range of
fluctuation than that of the ordinary mercurial barometer. The instrument
as proposed consists of a glass tube having at one end a flexible or elastic
chamber containing alcohol or other suitable liquid, and at the other a rigid
bulb or chamber containing air or æriform fluid, the two chambers communi-
cating by means of the tube. The elastic reservoir or chamber for containing
the liquid may be constructed of any form or material susceptible of a slight
degree of compression or distention by the action of fluid pressure, but I
prefer a disc-shaped metallic box having one or both sides corrugated. In
filling the instrument enough of the liquid used should be put in to fill the

page_text: clastic chamber and lower portion of the tube, air or æriform fluid being at
the same time admitted into the rigid bulb and upper portion of the tube.
The whole should then be hermetically sealed; the instrument will then
shew variations in atmospheric pressure by the rising or falling of the column
of liquid in the tube. In order to prevent the accuracy of the indications being
destroyed by the effect of variations of temperature, the elastic chamber must be
made of such a capacity in relation to its tensile force, and the expansibility
of the liquid used that the expansion and contraction of the latter caused
by fluctuations of temperature may produce variations in the tension of
the flexible sides of the chamber just sufficient to counteract the simul-
taneous variations in tension of the air in the bulb; and inasmuch as the
efficiency of this method of compensation depends on the liquid and air
chambers having an equal temperature, the instrument should be so constructed
that they may be equally exposed to changes of temperature, and as nearly as
possible equally sensitive to such changes. These relations being observed, no
change in the height of the column will be caused by changes of temperature,
while it will indicate very minute changes of atmospheric pressure. I also
propose to use an auxiliary spring of any convenient form acting from without
on the elastic chamber, either in the way of compression or distension, as may
be found convenient, which spring will serve the double purpose of more per-
fectly adjusting the compensation for temperature, and of afterwards regulating
the height of the column, for since the strength of the auxiliary spring is
so much added to the total tensile force of the chamber, by increasing or
diminishing that strength the due proportion between the capacity and tensile
force of the latter may be more easily attained; afterwards by increasing or
diminishing the tension of the spring by means of a screw and nut, or otherwise,
the height of the column of liquid may be regulated.
SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said William Haslett Mitchel in the Great Seal Patent Office on
the 8th October 1863.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, WILLIAM
HASLETT MITCHEL, of Hampstead, in the County of Middlesex, Gentleman,
send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Eighth day of April, in the year of our Lord Onc
thousand eight hundred and sixty-three, in the twenty-sixth year of Her reign,

page_text: did, for Herself, Her heirs and successors, give and grant unto me, the said
William Haslett Mitchel, Her special license that I, the said William Haslett
Mitchel, my executors, administrators, and assigns, or such others as I, the
said William Haslett Mitchel, my executors, administrators, and assigns, should
at any time agree with, and no others, from time to time and at all times
thereafter during the term therein expressed, should and lawfully might make,
use, exercise, and vend, within the United Kingdom of Great Britain and
Ireland, the Channel Islands, and Isle of Man, an Invention for "AN IMPROVED
CONSTRUCTION OF BAROMETER," upon the condition (amongst others) that I,
the said William Haslett Mitchel, by an instrument in writing under my
hand and seal, should particularly describe and ascertain the nature of the
said Invention, and in what manner the same was to be performed, and cause
the same to be filed in the Great Seal Patent Office within six calendar months
next and immediately after the date of the said Letters Patent.
NOW KNOW YE, that I, the said William Haslett Mitchel, do hereby
declare the nature of my said Invention, and in what manner the same is to
be performed, to be particularly described and ascertained in and by the
following statement, reference being had to the Drawing hereunto annexed,
and to the letters and figures marked thereon (that is to say) :-
This Invention relates to certain means for improving the construction of
that class of barometers known as "hermetic barometers," the object being to
perfect in a simple and inexpensive manner their adjustment, and regulate the
height of the column of indicating liquid.
In the accompanying Drawing I have shewn my improved instrument in
three forms, the same principle of construction being embodied in each. The
simplest form of instrument is shewn at Figures 1 and 2, Fig. 1 being a front
view, and Fig. 2 a sectional view thereof.
a is a tube of glass opening at one end into a slightly flexible chamber b.
containing alcohol or other suitable liquid, and expanding at the other into a
rigid bulb or chamber c containing air or æriform fluid; these two chambers
therefore communicate by means of the tube a. The form of the flexible
chamber and the material of which it is composed may be varied so long as
the chamber, is susceptible of soine degree of compression or distension; [
however prefer a disc-shaped glass or metallic box having one or both sides
corrugated. The flexible chamber band lower portion of the tube a being
illed with the liquid used, and air or æriform fluid being at the same time
admitted into the bulb c and upper portion of the tube, the chamber is sealed,
and the instrument will then show variations in atmospheric pressure by the
ising or failing of the column of liquid in the tube. In order to prevent the

page_text: accuracy of its indications being destroyed by the effect of the variations of
temperature, the liquid chamber must be made of such a capacity in relation
to the elastic force of its sides, and the rate of expansion of the liquid used, that
the expansion and contraction of the latter, due to the fluctuations of tempe-
rature, may produce variations in the tension of the flexible sides of the chamber
just sufficient to counteract the simultaneous variations in tension of the air
in the bulb, in which case the height of the column will remain unaffected by
temperature. In order to neutralize in this manner the effect of fluctuations
of temperature on the height of the column of liquid, it is necessary that there
should be a certain relation between the elastic force of the sides of the liquid
chamber, the amount of liquid used, its rate of expansion, and the actual tension
of the air in the bulb. With a view therefore to giving the liquid chamber
a definite uniform capacity, I make it in the form of a disc-shaped box of
sheet metal, the exact capacity proper for a given thickness of metal in the
sides, with the air in the bulb at an ordinary pressure, being readily ascertained
by experiment. Should the capacity be slightly in excess it may be corrected
by introducing some filling pieces of wire or other solid objects; if corrected
in this way glass chambers may be used. As a means of perfecting the
adjustment of the instrument after it has been sealed up, I apply to the
chamber 6 an auxiliary spring d, which nets from without on the chamber, and
either in the way of compression or distension, as may be found convenient.
This spring d is represented as a curved plate having its ends resting on one
side of the chamber near the periphery. Through a hole in the centre of the
spring passes freely a screw c, which is secured by a rivet head to the adjacent
side of the chamber b. Fitting on to this screw is a nut f, by means of which
the spring and adjacent side of the chamber are connected firmly together, and
increased rigidity is thereby imparted to the side of the chamber. This nut
may also be used to draw the spring and flexible side of the chamber towards
one another, thereby enlarging the capacity of the chamber and reducing the
column of liquid to a determinate height. In Figures 3 and 4 a clamping
spring g is shewn for adjusting the capacity of the chamber. In this case the
action of the spring is to compress the chamber, the amount of compression
being regulated by a screw h which bears upon the side of the chamber. To
admit of the application of the spring, I make the liquid chamber of a capacity
insufficient to contain the amount of liquid requisite by its expansion to
maintain the column of liquid at the regulated height against the adverse
expansion of the confined air. The instrument when sealed is subjected to a
known rise of temperature, the amount of error noted, and a spring added o
strength sufficient to correct it. By means of the spring the height of the

page_text: column of liquid may be regulated, its tension being for that purpose increased
or diminished by means of the screw. It will be obvious that any convenient
form of spring may be used for this purpose. Figures 5 and 6 shew the
improved barometer adapted for the pocket. In this instance the chamber b
instead of being pendent from the tube a is set parallel therewith, and con-
nected to the tube by an elbow joint. The principle of construction is otherwise
similar (as will be understood) to the barometer shewn at Figures 1
and 2.
Having now set forth the nature of the Invention of "An Improved Con-
struction of Barometer," and explained the manner of carrying the same into
effect, I wish it to be understood that under the above in part recited
Letters Patent I claim constructing hermetic barometers in the manner above
described.
In witness whereof, I, the said William Haslett Mitchel, have hereunto
set my hand and seal, the Eighth day of October, in the year of our
Lord One thousand eight hundred and sixty-three.
Witness,
J. W. MOFFATT,
66, Chancery Lane.
W. H. MITCHEL. (L.s.)