288 Dr Searle, Some methods of measuring 
§ 4. Measurement of surface tension of water. The balance 
may also be used to determine the surface tension of water or any 
other transparent liquid which will not form persistent films. A 
thin rectangular glass plate is held in a clip (Fig. 2) by which it 
is suspended below the scale pan instead of the wire frame. The 
glass slips sold for microscope slides are convenient. The plate is 
adjusted in the clip so that when it is suspended its lower edge 
is horizontal. The plate is then allowed to dip into the liquid in 
the beaker and the balance is raised until the lower edge of the 
plate is exactly in the plane of the undisturbed part of the 
surface of the liquid; the fine adjustment is conveniently made 
by the levelling screw in the base of the instrument. 
The scale reading of the index is now taken and then the 
beaker of liquid is removed and the plate is dried by filter paper. 
A mass m grms. is then placed in the pan to bring the index to 
the first reading. If the length of the plate be / cm. and its 
thickness be a cm., the surface tension is given by 
ul 
= See) dynes per cm. 
If water be used, the surface should be free from grease. 
The beaker should be cleaned with potash and should be filled 
with water freshly drawn from the tap. The glass plate should 
also be cleaned with potash. 
§ 5. Thread method. The surface tension of a soap solution 
can be found by this method with very simple apparatus. 
Fig. 3. 
On the horizontal arm QR (Fig. 3) of a bent glass rod PQA 
slide two rings A, B. Through eyes on these rings passes a 
thread whose ends are attached to a glass rod CD. By adjusting 
the rings and the thread the distances AB and CD can be made 
equal, and the rod can be made to hang with its axis horizontal. 
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