The mercurial barometer in use today is practically the same as that
invented by Torricelli. Of course, many changes have been made in the
case containing the tube of mercury, adding to its attractiveness, but
the principle remains the same.
The standard mercurial barometer consists of a straight glass tube about
thirty-two or thirty-three inches in length, hermetically sealed at one
end. The tube is of half-inch bore and is filled with chemically pure
mercury, which has been boiled in the tube to insure the total exclusion
of all air and moisture. After the tube has been filled, the open end is
immersed in a cistern of mercury. Upon immersion the mercury drops in
the tube to a height of 29.92 inches at sea level, or until
counterbalanced by the weight of the surrounding atmosphere pressing
upon the surface of the mercury in the cistern. The space in the top of
the tube is a perfect vacuum and is called the Torricellian vacuum.
The glass mercury tube is enclosed in a brass case. About two inches
from the top of the case is an opening extending down the front and back
for a distance of about eight inches. On each side of this opening is a
graduated scale, one side being in inches and the other graduated in
centimeters. The opening is fitted with a sliding vernier scale
graduated in millimeters, thus permitting the reading of changes in the
height of the mercury column most accurately, as the sliding vernier may
be adjusted to the level of the mercury by means of a thumb screw fitted
on the side of the case. The cistern containing the mercury is of glass,
with a soft leather or chamois bottom and an adjusting screw, used to
raise or lower the level of the mercury, so that it just comes in
contact with a small ivory point, inserted in the top of the cistern,
and which is used to mark the zero of the scale. Observations of the
changes in the atmospheric pressure should be taken at regular
intervals, and it is necessary to adjust the height of the mercury in
the cistern before each observation. This is done by bringing the ivory
point in contact with the level of the mercury and then bringing the
vernier scale absolutely level with the top of the column of mercury in
the tube, and then take the reading.
The mercurial barometer is a very delicate instrument and when once
placed in the desired position should not be moved. Care should be taken
that the room in which the barometer is placed is of nearly uniform
temperature, for if the temperature at the top of the barometer is
different than the temperature at the bottom, of course there will be an
effect produced on the changes in the mercury column. All other
barometers are set by the mercurial.
THE ANEROID BAROMETER (Fig. 42).
The aneroid barometer is so constructed that it contains no liquid
whatever, and thus derives its name from the Greek compound word
“aneroid,” meaning “without fluid.”
The essential parts of the instrument are a metallic case from which the
air has been exhausted, and which contains a spring. The case of elastic
metal is fastened to a base plate at the bottom and to the spring at the
top. The pressure of the atmosphere causes the case to expand and
contract, thus affecting the spring, which is connected to a needle or
dial, causing the dial to move around on the scale on the face of the
instrument and record the changes. The scale is marked off in inches
from 28 to 31, and besides a brass hand or pointer, used to designate
the changes in the atmospheric pressure, there is a small index hand to
set over the needle so that the amount of change in a certain period is
easily known on consulting the instrument.
[Illustration:
_Courtesy Julien Friez & Sons, Baltimore, Md._
Fig. 45
]
The dial of the barometer is marked with the words “Fair,” “Change,” and
“Rain,” etc., but these words have no significance, and should be
disregarded. For instance, 29½ is marked “Change”; 30, “Fair”; 31, “Very
dry”; 28½, “Rain.” If the barometer, which has been standing at 30.9,
suddenly drops down to 29.9, this is positive indication that a storm is
approaching, with strong winds, yet, according to the dial on the
aneroid, the reading would be “Fair.” If the barometer were standing at
28 and rose to 29, this would actually indicate approach of cold, dry
weather, and yet on the dial it reads “Rain.” This simply goes to show
that the readings on the dial are of no significance whatsoever, and are
not to be relied upon.
The aneroid is not as accurate an instrument as the mercurial, so should
be checked up occasionally with the mercurial barometer.
[Illustration:
_Courtesy Taylor Instrument Companies Rochester, N. Y._
Fig. 46
]
The aneroid type of barometer is also used in altitude work, but must be
compensated before using.
This type of barometer possesses several advantages over the mercurial
in that it is portable and therefore used for altitude work; at sea it
is used because there is no fluid to become unsettled by the motion of
the vessel; it is used also in observatory work because the action is
quicker than the mercurial barometer action, and sudden changes likely
to occur are indicated.