the surface tension of soap films 287 
of the index will change slightly, but will reach a steady value 
after a short time. This steady reading is recorded. The 
reading must be taken to 7, mm.; a short focus lens mounted 
in a clip may be used to give the necessary optical assistance. 
The film is now broken. The arm then rises but is brought 
back to the position it had when the film was intact by placing 
a mass m grms. in the pan. If necessary, the exact mass required 
is obtained by interpolation from two scale readings, the mass 
being a little too large for one reading and a little too small for 
_the other. A “rider” may also be used to obtain the fine adjust- 
ment. If its mass be r grms., its effect is equivalent to that of 
a mass rv/d grms. placed in the pan, where d and «@ are the 
distances from the axis of the torsion wire to the notch and to the 
point of suspension of the rider. Since the volume of the wire 
below the surface of the solution is the same as when the film was 
intact, the upward thrust of the solution is the same in each case. 
Hence the weight, mg dynes, of the mass in the pan is equal 
to the force which was exerted by the film. Thus 
mg =2T1 
yr. MG 
or i 31 
where J is the distance HF (Fig. 1) between the points where 
the frame cuts the solution. This is the shortest distance 
between the two circles in which the plane of the surface inter- 
sects the two wires. 
After each pair of readings, the height of the torsion balance 
may be slightly changed or a small mass may be placed in the pan. 
In this way a number of independent readings may be obtained. 
§ 3. Practical example. The following results were obtained 
in an experiment : 
Width of frame=/=8'00cm. Temperature about 19°C, 
GIPIDESS [EW GIN, .504ononc00vebon0b00 (1), 
Readings with film broken 
Reading with 
film unbroken | 
0:40 grm.in pan 0°45 grm. in pan m 
3°83 cm. 3°79 cm. 3°85 cm, 0°433 grm. 
3°87 3°82 3°88 0-442 
3°90 3°85 3°90 0-450 
4:00 3°94 4-00 0°450 
Mean value of m=0°444 grm. 
mg  0'444x 981 
H NE) 
Ta De hae MORES ‘00 
= 27°22 dynes per cm. 
