Dr Searle, Some methods of measuring the surface tension, etc. 285 
Some methods, of measuring the surface tension of soap films. 
By G. F. C. SEARLE, Sce.D., F.R.S., University Lecturer in Ex- 
perimental Physics, Fellow of Peterhouse. 
[Read 19 May 1913.] 
§1. Introduction. The following paper gives an account of 
some methods employed in my practical class at the Cavendish 
Laboratory for the measurement of the surface tension of films 
of soap solution. The first two methods have been in use for 
some years, but I have included them in the hope of making the 
paper more useful to teachers of practical physics. The apparatus 
may, without loss of efficiency, be constructed in quite “ home- 
made” style or may be built up of elements at hand in most 
laboratories. 
§2. Torsion balance method. Let ABCD (Fig. 1) be a 
rectangular frame of thin wire, the plane of the rectangle being 
Fig. 1. 
vertical. If the frame be dipped into a soap solution and then 
be partially withdrawn so that the horizontal surface of the 
solution cuts the frame at # and F, a film will be formed, which 
will fill the area ABFE. This film will pull the frame down- 
wards. If the surface tension of the solution be 7 dynes per 
centimetre*, and if the distance HF be J cm., the downward pull 
on the frame will be 277 dynes, each surface of the film con- 
tributing 7 dynes. If the downward pull be measured, the 
surface tension can be calculated. 
The pull of the soap film is easily measured by aid of the 
simple torsion balance shown in Fig. 2. This was designed, in 
conjunction with Mr W. G. Pye, as a more convenient form of the 
apparatus originally constructed at the Cavendish Laboratory. 
The base of the balance is a tripod stand furnished with a 
levelling screw. From this stand rises an adjustable vertical rod 
carrying a stiff metal frame, across which is stretched a torsion 
wire; the tension of the wire can be adjusted by a screw. A 
double-ended beam is attached to the wire. The long arm of the 
* So many elastic constants are expressed in dynes per square centimetre that 
students easily fall into the error of stating surface tensions in dynes per square cm. 
VOL. XVII. PT. III. 19 
