190 



CHEMICAL TECHNIQUES 



large enough to hold it) fused to a Pyrex tripod. This enables easier 

 replacement of fibers. The Pyrex sleeve should lean toward the 

 front of the instrument at an angle of a little less than 90° to the 

 plane of the tripod. The free end of the fiber (D) is bent into a tiny 

 V in a plane at right angles to the fiber axis. Without a load, the 

 free end of the fiber should be 12-15 cm. above the tripod. The 

 tripod is mounted inside a metal cylinder (E), a gallon tin can will 

 do or a smaller instrument can be made to fit into a smaller can. The 

 open front of the cylinder is fitted with a removable glass plate {F). 



tz^ 



_^.w 



•10 cm- 



N 



Fig. 74. Quartz-fiber balance. 

 From Lowry (194V 



The tripod is fixed in place with DeKhotinsky cement, and the 

 cylinder is mounted rigidly on a heavy wooden block. A cathetometer 

 (Q) , reading to 0.01 mm., is used to observe the positions of an 

 arbitrary point on the fiber tip when weights are applied. Illumina- 

 tion of the interior of the cylinder can be enhanced by removing 

 the back end of the can and replacing it with a plate of glass. 

 Electrostatic shielding can be increased by lining the inside surfaces 

 of the glass plates with metal foil from the center of which strips 

 have been cut to act as windows. For less accurate measurements 

 without a cathetometer, a narrow ribbon of graph paper running 

 down the center of the front window can serve for the indication of 

 the displacement of the end of the fiber. 



