71 



My first frames were made by cutting the mica sheet as it lay under a steel 

 rule upon a piece of plate glass. I afterwards had made two heavy steel plates of 

 the exact shape of the frame desired. The inner surface of each plate was ground 

 plane with emery dust upon plate glass. A sheet of mica was clamped between 

 them and cut to their dimensions. The advantages of frames made in this way 

 are : 



The steel plates are accurately ground; the frames are correspondingly 

 regular. 



The mica does not split along the cut edge. 



The edge is of the same thickness as the plate itself; there is no bur. 

 Very thin frames are easily made, but it is difficult to work with them 

 when they are much less than 0.002 cm. thick. 

 A difficulty experienced with the mica frame, as also with those of platinum 

 and aluminum, is that the fluid does not readily and ecjually wet all portions of 

 the surface. It has a tendency to collect in drops, rendering the after-weighing 

 uncertain. This difficulty was entirely overcome by roughing the surface (dark- 

 ened in Fig. 4) of the plate near the edge by rubbing very lightly with the finest 

 French emery paper. Both weights could then be taken again and again with a 

 variation of only a few hundredths of a milligram. 



The advantages claimed for the mica frame are as follows : 



1. They are easily made, and do not require careful handling. 



2. They are of even thickness, with straight edges and square corners. Hence 

 the film length is not so uncertain as with glass frames. 



3. They can be made less than one-tenth of the thickness of a glass frame, 

 reducing the correction correspondingly. Table I gives the relative corrections 

 for glass and mica frames, obtained by determining the maximum weight for a 

 soap solution, and then weighing the film itself. The film weight divided by 

 twice the length of the frame gives the surface-tension. But with many liquids it 

 is impossible to obtain the film weight, as the film breaks immediately after it is 

 formed. The maximum weight can be determined in almost every case, and the 

 film weight by correction. It is evident that a slight error in the value of this 

 correction will be lessened by reducing the total correction, as is done by using 

 the mica frame. 



