294. Dr Searle, Some methods of measuring 
effects of its heat or a mirror reflecting the sky light may be used 
to give the necessary illumination. 
A film is formed on the ring by dipping a card into the 
solution and then drawing the edge of the card gently across the 
ring. 
“The position of the zero of the glass scale relative to the plane 
of the rim of the ring is found by placing a steel scale of known 
width on the rim with the plane of the scale vertical and then 
observing the reading of the upper edge of the steel scale on the 
glass scale. 
An alternative method of determining the height of the vertex 
of the film is to use a horizontal microscope provided with a 
vertical motion and to “set” the microscope first on the vertex of 
the film and then on the rim of the ring. The difference of the 
readings gives the distance h. 
§ 9. Practical details. A series of measurements may be 
taken by varying the counterpoise of the gasometer, and deter- 
mining 7, the radius of the spherical film, in each case. The value 
of Mr, where M cm. is the difference of level of the manometer 
columns, is found in each case, and the mean of these values is 
used in calculating the surface tension. Since the pressure in the 
gasometer slowly diminishes, the observations for r and M should 
be made as nearly as possible simultaneously. 
$10. Geometry of the curved film. When a curved film is 
formed on a circular rim by a pressure excess p, it must be a. 
surface of revolution about the axis of the rim, and this surface is 
easily shown to be part of a sphere. Let VMO (Fig. 8) be the axis 
Fig. 8. 
of the rim of the cup, and V the vertex of the film; the figure is 
a section of the system by a plane through VO. Let NMN’ be | 
the trace of a plane normal to VO and let MN=2 and VM = Yo 
Let the tangent at WV to the section of the film make the angle 0 — 
with MN. Since the force on the film WV’ due to the surface 
tension on both sides of the film acting along the circle WN’, in 
which the plane VMN’ cuts the film, is equal to the force due to 
the pressure excess p acting on the circular area of radius MN, 
the equation of equilibrium is 
2T sin 0. 270= >. 1a?, 
