the surface tension of soap films 291 
If the length of each thread, i.e. the length of the are BHD, 
be J cm., the radius can be found from an approximate formula (6). 
Thus, if the angle BHO be ¢, we have 6=1/2p. But 
sige Pokus \=$ ( ales 
Patiresa7-)73 U-tt) 
i? B 
=a eo 
H aces 
ence p=3-( Se) 
The first approximation is p =/?/8x. Using this in the term 
[?/48p, we have as a second approximation 
xv 
—=1-—cos =1-(1 
: $ 
P 4 a Pa 
p-g(l-gpet-)=g-gt SIOIOIGIOOIOIG (6). 
§6. Practical example. The following is a record of an 
experiment by Mr C. E. Simmons. 
Distance HF between threads when vertical=a=1-68 cm. 
Minimum distance G'H between threads when curved =6=1°'15 cm. 
Vertical distance BD when film is unbroken =h=7°40 cm. 
Mass of glass rod CD=m=2'94 germs. 
Hence x= (a—b)=0°265 cm., 
Be 2 AO $0265 
P82 ' 2 8x0265' 2 
Hence, by (4), 
T= mg 294x981 
~ 2b+4p 2°304+103°84 
and = 25°83 +0:13 = 25°96 em. 
= 27°17 dynes per cm. 
87. Viscosity potentiometer method. In this method the 
pressure excess due to a spherical soap film is measured by 
aid of what may be called a “viscosity potentiometer.” Air from 
a gasometer G (Fig. 6) flows through two tubes AB, CD, which 
are connected in series by the joint BC. The pressure at A 
Fig. 6. 
