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- The following instructions have been taken, with minor editing,
from: Minor, Ralph S. Physical Measurements: A Laboratory
Manual in General Physics for Colleges. Part II: Heat, Mechanics
and Properties of Matter. Berkeley, Calif. (1919) pp. 23-26.
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- Copyright © 1998 Richard A. Paselk
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- 5. SURFACE TENSION BY JOLLY'S BALANCE.
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- To obtain a direct measure of the surface tension of a
liquid by balancing it against the tension in a stretched spring.
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- A wire rectangle is hung from the spring of a Jolly's balance
and allowed to dip in a soap solution which forms a film across
the rectangle. When equilibrium is established, the force due
to surface tension in the two surfaces of the film must just
balance the tension in the spring. By knowing the force which
will stretch the spring the same amount, we have a measure of
the total force due to surface tension. If T is the value
of the surface tension in dynes per centimeter width of the film,
l the width in centimeters of the rectangle along the
surface of the liquid, and F the force in dynes exerted
by the spring,
- F=2lT.
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- The force F in dynes is equal to 980 m, where
m is the mass in grams whose weight will stretch the spring
the given amount and 980 is approximately the number of dynes
of force which the earth exerts on 1 gm. Knowing m and
l, the value of T can be calculated.
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- Bare wire, pincers, thread and a piece of emery cloth are
provitled for the construction of the rectangles. The greatest
care must be taken to have the beaker and rectangles clean. Do
not touch with the fingers the inside of the beaker, the liquid,
or the part of the rectangle on which the film is formed, for
a slightt trace of grease will very greatly decrease the surface
tension of water.
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- (a) Having first cleaned the pincers and the wire
with emery cloth construct three rectangles about 2, 4, and 6
cm. wide respectively. Make the rectangles to approximate size
and determine their exact width after you have finished using
them. These should have the form of a staple with square corners
and legs from 3 to 5 cm. long. Suspend a rectangle, 2 cm. wide,
from the spring, and let it be partially immersed in a beaker
of soap-solution. Read the extension of the spring when there
is no film in the rectangle, and again with a film across it.
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- Repeat until consistent results are obtained, and average.
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- Repeat these measurements, using rectangles 4 cm. and 6 cm.
wide.
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- Determine by trial whether the force exerted by the film
depends on the length of the film, measured parallel to the direction
of the force, i.e, parallel to the height to which the film is
drawn.
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- (b) Calibrate the balance by observing the extension
produced by known weights. Use extensions equal to or somewhat
larger than those obtained with the film.
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- (c) Make a new rectangle, 4 cm. wide; clean the beaker
thoroughly; and repeat (a) with water fresh from the tap. As
a film of no appreciable height will form with water, take the
reading of the balance without the film when the underside of
the upper wire of the rectangle is just above the surface of
the water and not in contact with it; and again, after immersing
the upper wire of the rectangle so as to wet it, take a reading
when it breaks sway from the surface.
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- Repeat until consistent results are obtained, and average.
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- (d) Repeat (c), using water at 50°C. or
higher.
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- (e) Repeat (c), using alcohol.
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- (f) From the data taken in (a), state how the
total tension in the film varies with its width. Calculate the
surface tension, T, in dynes per cm., for the liquids
used in (a), (c), (d), and (e). Present
your results in tabular form.
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- © R. Paselk
- Last modified 22 July 2000
