SYPHON TUBE BAROMETERS.
=30. Principle of.=--If some mercury, or any other fluid, be poured into a
tube of glass, bent in the form of =U=, and open at both ends, it will
rise to the same height in both limbs, the tube being held vertically. If
mercury be poured in first, and then water upon it at one end, these
liquids will not come to the same level; the water will stand much higher
than the mercury. If the height of the mercury, above the line of meeting
of the fluids, be one inch, that of the water will be about
thirteen-and-a-half inches. The explanation of this is, that the two
columns balance each other. The pressure of the atmosphere in each limb is
precisely similar; but the one column stands so much higher than the
other, because the fluid of which it is composed is so much lighter, bulk
for bulk, than the other. If one end of the tube be hermetically closed,
the other limb be cut off within a few inches of the bend, and the tube
carefully filled with mercury; by placing it in a vertical position, the
mercury will fall, if the closed limb be long enough, until it is about
thirty inches higher than that in the exposed limb, where it will remain.
Here the atmosphere presses upon the short column; but not upon the long
one. It is this pressure, therefore, which maintains the difference of
level. In fact, it forms a barometer without a cistern, the short limb
answering the purpose of a cistern. The first barometers on this principle
were devised by the celebrated philosopher, Dr. Hook, as described in the
next section.
31. DIAL, OR WHEEL BAROMETERS.
The familiar household "Weather Glasses" are barometers on the syphon
principle. The portions of the two limbs through which the mercury will
rise and fall with the varying pressure of the atmosphere are made of
precisely the same diameter; while the part between them is contracted. On
the mercury, in the exposed limb, rests a round float of ivory or glass;
to this a string is attached and passed over and around a brass pulley,
the other end carrying another lighter weight. The weight resting on the
mercury rises and falls with it. On the spindle of the pulley, which
passes through the frame and centre of the dial-plate, is fixed a light
steel hand, which revolves as the pulley turns round. When the mercury
falls for a decrease of atmospheric pressure, it rises by the same
quantity in the short tube, and pushes up the float, the counterpoise
falls, and thus moves the hand or pointer to the left. When the pressure
increases, the pointer is drawn in a similar manner to the right.
[Illustration: Fig. 18.]
[Illustration: Fig. 19.]
[Illustration: Fig. 20.]
The dials are generally made of metal silvered over or enamelled, but
porcelain may be used. If the circumference of the pulley, or "wheel," be
two inches, it will revolve once for an alteration of level amounting to
two inches in each tube, or four inches in the height of the barometric
column; and as the dial may be from twenty to thirty-six inches in
circumference, five to nine inches on the graduated scale corresponds to
one inch of the column; and hence the sub-divisions are distinctly
perceptible, and a vernier is not necessary.
The motion of the pointer alone is visible; and a mahogany, or rosewood,
frame, supports, covers, and renders the instrument ornamental and
portable. In the back of the frame is a hinged door, which covers the
cavity containing the tube and fixtures. The dial is covered by a glass in
a brass rim, similar to a clock face. A brass index, working over the
dial, moveable by a key or button, may be applied, and will serve to
register the position of the hand when last observed. These instruments
are usually fitted with a thermometer, and a spirit level; the latter for
the purpose of getting the instrument perfectly vertical. They sometimes
have, in addition, a hygrometer, a sympiesometer, an aneroid, a mirror, or
a clock, &c., singly or combined. The frame admits of much variety of
style and decoration. It may be carved or inlaid. The usual adjustment of
scale is suited for localities at no considerable elevation above the
sea. Accordingly, being commercial articles, they have been found
frequently quite out of place. When intended for use at high elevations,
they should have a special adjustment of scale. As household instruments
they are serviceable, and ornamental. But the supply-and-demand principle
upon which they are sold, has entailed upon those issued by inferior
makers a generally bad adjustment of scale. The illustrations are those of
ordinary designs.
[Illustration: Fig. 21.]
[Illustration: Fig. 22.]
[Illustration: Fig. 23.]
Dial barometers required for transmission to distant parts, as India and
the Colonies, are furnished with a steel stop-cock, to render them
portable more effectually than can be done by the method of _plugging_ the
tube.
32. STANDARD SYPHON BAROMETER.
Fig. 24 represents the most accurate form of the Gay Lussac barometer. The
short limb is closed at the top, after the mercury is introduced, and a
small lateral puncture is made at _a_, which is covered over with a
substance which permits the access of air, but prevents the escape of any
mercury when the instrument is packed for travelling. The bent part of the
tube is contracted to a capillary bore; and just above this, in the long
limb, is placed the air-trap, already described (see p. 17), and here
illustrated (fig. 25). When reversed, as it must be for portability, the
capillary attraction keeps the mercury in the long branch. Should the
mercury of the short column get detached, some small quantity of air _may_
pass; but it will be arrested at the pipette, and will not vitiate the
length of the barometric column. It can be easily expelled by gently
shaking or tapping the instrument before suspending it for observation. In
the illustration, the zero of the scale is placed at Z, near the middle of
the tube; and the graduations extend above and below. In making an
observation, it is necessary to take the reading ZA on the long branch,
and ZB on the short one. The sum of the two gives the height of the
barometer. The zero of the scale in some instruments is placed low down,
so as to require the difference of the two readings to be taken. A
thermometer is attached to the frame as usual.
[Illustration: Fig. 24.]
[Illustration: Fig. 25.]
These instruments can be very accurately graduated, and are very exact in
their indications, provided great care has been exercised in selecting the
tubes, which must be of the same calibre throughout the parts destined to
measure the variations of atmospheric pressure. They should be suspended
so as to insure their hanging vertically.
The syphon barometer does not require correction for capillarity nor for
capacity, as each surface of the mercury is equally depressed by capillary
attraction, and the quantity of mercury which falls from the long limb of
the tube occupies the same length in the short one. The barometric height
must, however, be corrected for temperature, as in the cistern barometer.
Tables containing the temperature corrections to be applied to barometer
readings for scales engraved on the glass tube, or on brass or wood
frames, are published.