the center plate at approximately 88 turns per inch. The wire is then 

 silver- coated on one side of the plate, giving in effect a series of 

 thermocouple junctions on opposite sides of the plate. 



Aluminum cover plates are mounted on both sides of the thermopile. 

 These plates increase the thermal capacity without appreciably increas- 

 ing the thermal resistance, thereby damping out minor variations in 

 heat flow caused by fluctuations in the air stream. The cover plates 

 also give additional strength and weather resistance. The upper 

 aluminum plate is painted black to absorb the incident radiation, and 

 the lower aluminum plate is highly polished to reduce the emissivity 

 and absorbtivity to the lowest possible values. When the instrument is 

 used as a net exchange radiometer, both the upper and lower aluminum 

 plates are painted black. 



Since long wave radiation will not penetrate glass, no protective 

 coverings are put over the sensitive surfaces. The sensing unit is 

 mounted in the air stream from a small blower to maintain uniform 

 values of the unit thermal resistance from both meter surfaces. The 

 air stream also prevents the deposit of dust or dew on the meter 

 surface. 



The correlation between radiometer and pyrheliometer readings 

 is reportedly high. When the radiometer and pyrheliometer are used 

 together, and since the pyrheliometer is sensitive only to short wave 

 radiation, the long and short wave components can be separated from 

 the total radiation by subtracting the pyrheliometer reading from 

 the radiometer reading. 



The use of the radiometer at sea remains to be proven. Past 

 attempts at use have resulted in erratic readings which are believed 

 caused by irregular winds striking the sensitive surface. Sea spray 

 and salt deposits on the sensitive surface likewise change the calibra- 

 tion from time to time. 



The Geir and Dunkle radiometer has been used over fresh water 

 and snow and in dusty areas with a high degree of dependability and 

 has required minimum maintenance over prolonged periods. 



2. Instruments used under water 



a. Secchi disc 



The Secchi disc probably has been the most widely used 

 device for measuring the clarity of ocean waters. It consists of a 



VIII- 3 



