286 Dr Searle, Some methods of measuring 
beam is pointed to serve as an index, and this index point moves — 
near a vertical scale divided to millimetres; the short arm carries 
an adjustable counterpoise for bringing the beam to a horizontal 
position. To secure a good connexion of the beam to the wire, 
the beam is clamped to a short metal tube of small bore, through 
which the wire passes and to which the wire is soldered. Near 
. 
the pointed end of the beam is cut a small notch which serves — 
to define the position of a hook supporting a small scale pan. 
Below the scale pan hangs the wire frame on which the film is 
formed. This frame is about 8 cm. in length and 3 cm. in 
height. 
The sensitivity of the torsion balance depends upon the 
thickness of the torsion wire; by using wires of different thick- 
nesses a wide range of sensitivity can be covered. For the 
present purpose, it 1s convenient to use a torsion wire such that 
one deci-gramme in the scale pan gives a deflexion of about 
2°5 mm. 
A beaker containing soap solution is placed so that the frame 
dips into it, and the height of the torsion balance is adjusted 
so that when the frame is drawn down by the film, the film is 
one or two centimetres in height. There must be enough solution 
in the beaker to allow the frame to be completely immersed when 
necessary. 
The measurements are taken as follows:—The end of the 
balance arm is depressed so as to completely immerse the frame. 
The arm is then allowed to rise, when a film will be formed 
between the emergent part of the frame and the horizontal 
surface of the solution. The solution will then begin to drain 
off from the wire and from the film itself, and the scale reading 
