GENERAL APPARATUS AND MANIPULATION 189 



of the pipettes thus formed are bent at right angles about 5 mm. 

 from the small end. Care must be taken that the ends are cleanly 

 and squarely cut. A mercury leveling bulb connected to the pipette 

 with rubber tubing may be used for filling and emptying. Small 

 air bubbles sometimes cling to the cell walls or to the electrodes when 

 the cell is filled. This difficulty is overcome as a rule by withdrawing 

 the fluid into the pipette and refilling the cell more slowly. 



The circuit used by Bayliss and Walker was the standard Kohl- 

 rausch bridge fed from a 1000 cycle audiofrequency generator. The 

 null point was determined with head phones in the usual way. Be- 

 cause of the small size of the electrodes and the necessity of drying 

 them, the null point tends to be flat. A 0.01 /xF. condenser placed 

 across the standard resistance makes the null point sharp. The 

 practice is to adjust the resistance until minimum sound intensity 

 is obtained with the slide wire at midpoint, or until a sharp increase 

 in intensity occurs symmetrically on each side of it. 



I. BALANCES 



The development of the instrumentation for the weighing of very 

 small amounts has been thoroughly review-ed by Gorbach (1936). 

 The commercial balances, including the torsion balances of Roller 

 Smith Co., which are sensitive down to about 2 fig., require no 

 comment here. The quartz fiber balances, which are considerably 

 more sensitive, are particularly useful in histochemical work and 

 these will be considered in detail.* 



Quartz Fiber Balance. Lowry (1941) designed a simple and 

 serviceable quartz fiber balance that can handle a maximum load 

 of 200-300 fig., and that has a sensitivity of about 0.03 fig. and a 

 reproducibility of 0.1 jug. The functioning of the instrument (Fig. 

 74) depends on the measurement of the bending of a horizontal 

 hollow quartz fiber when a weight is attached to its free end. The 

 fiber (A), about 20 cm. long, is drawn from narrow quartz tubing. 

 One end of the fiber is fused at B to a low tripod (C) made of 1-2 

 mm. quartz rod. Instead of fusing the fiber to a quartz tripod, the 

 end can be inserted into a short Pyrex sleeve (having a lumen just 



* Since this writing a new quartz fiber balance has been described by Kirk 

 et al.; a "Cartesian-diver" balance has been announced by Zeuthen (Bibliog- 

 raphy Appendix, Refs. 39, 49, and 54.) 



