52 Applied Biophysics 



the central portion of the dip on opposite sides of the vessel. 

 At each end of the clip, there is a thermocouple differentially 

 connected. These make contact with the vessel wall. The pass- 

 age of the radio-frequency current through the blood stream 

 warms it slightly. The temperature difiference, which varies 

 inversely with the blood flow rate, is read electrically with a 

 sensitive galvanometer. Calibration of the instrument is done 

 by measuring the radio-frequency current used and adjusting 

 a comparison resistance to take the same current, thus permitting 

 the dissipated wattage to be estimated. The constants of the 

 blood vessel clips are readily fixed by the application of a simple 

 formula. A graph is obtained which permits this nondestructive 

 instrument to be used almost as easily as a direct-reading 

 mechanical flow meter. This method has been improved upon 

 by Essex, Herrick, Baldes, and Mann ^'^ and applied even to 

 the coronary circulation. 



Photocells 



Light-sensitive devices have been responsible for some of 

 the more recent developments of control engineering, as well as 

 of sound reproduction and optical determination. There are 

 three main types of photocell, the photoconductive, the photo- 

 electric and the photovoltaic. 



Photoelectric colorimeters are being used in many laboratories. 

 They can be applied to practically every colorimetric problem, 

 from the simple evaluation of intrinsic color at selected portions 

 of the visible spectrum, to the more complex requirements of 

 the analytical chemist. 



A fall in hemoglobin level is one readily detected sign of 

 incipient malnutrition. Another use for a rapid hemoglobin- 

 ometer would be in assessing minor degrees of anemia among 

 blood donors. The photoelectric hemoglobinometer is more ac- 

 curate than the visual method. In these, as green light is 

 absorbed by a red solution (of oxyhemoglobin), a constant 

 source of light is used together with an appropriate green filter 



