274 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 8 



allowing the surface tension of the medium to pull the drop from the capillary 

 tip. 



The relative density of water and the medium vary so greatly with tem- 

 perature that the temperature must be kept constant to ±0.001°C. This is 

 accomplished by mounting the tube in a well-stirred water bath of about 

 100 liters capacity and carefully controlling the bath temperature. Some 



form of automatic temperature regula- 

 tion is required. Baths with the 

 required control are available com- 

 mercially. Many designs for thermo- 

 stats have been published [62-68]. If 

 the bath temperature is to be above room 

 temperature, a temperature-sensitive 

 element such as a volume of mercury 

 with a large surface-volume ratio or a 

 bimetallic strip may be used to control 

 the current of an immersion heater 

 through a relay or thyraton circuit. In 

 a thermostat that uses a relay, the dis- 

 continuous input is the fundamental 

 obstacle to the attainment of perfect 

 constancy of temperature. Amplifier 

 devices may be used to give a continuous 

 but variable input, but these also have 

 a finite sensitivity. The Guoy principle, 

 applying a vertical oscillating motion to 

 the contacting wire of an expansion 

 thermoregulator, provides an intermedi- 

 ate type of control between simple on- 

 and-off control and continuous control 

 and has been used to hold the tempera- 

 ture of a 100-liter water bath constant 

 within + 0.0002°C for several hours [66]. 

 There is some advantage in having the 

 bath temperature below room temperature [56]. This may be effected with 

 the same kind of thermoregulators, but by controlling the flow of cold water 

 through the bath instead of controlling a heater. Many difficulties are 

 avoided by having the apparatus in a constant-temperature room and oper- 

 ating near room temperature. The multiple-enclosure principle of Tian 

 may also be used to decrease the effects of ambient variations [68,69]. 



Under the conditions used in this method, the rate of fall does not follow 

 Stokes' law perfectly, and it is necessary to establish a calibration curve by 



Fig. 58. Micropipette. A, stop; B, 

 bracket; C, pinion; D, stand; E, micro- 

 meter; F, plunger (}4.6 m - diameter 

 drill rod); G, packing; H, rubber washer; 

 /, lead washer; A", barrel; L, capillary. 



