Humboldt State University ® Department of Chemistry

Robert A. Paselk Scientific Instrument Museum

 
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.
 
Copyright © 1998 Richard A. Paselk
 

 
 
5. SURFACE TENSION BY JOLLY'S BALANCE.
 
To obtain a direct measure of the surface tension of a liquid by balancing it against the tension in a stretched spring.
 
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.
 
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.
 
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.
 
(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.
 
Repeat until consistent results are obtained, and average.
 
Repeat these measurements, using rectangles 4 cm. and 6 cm. wide.
 
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.
 
(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.
 
(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.
 
Repeat until consistent results are obtained, and average.
 
(d) Repeat (c), using water at 50°C. or higher.
 
(e) Repeat (c), using alcohol.
 
(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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Last modified 22 July 2000