Figure ll*. Checking voltage gradients within electrode array with 

 probe and vacuum tube voltmeter. 



selected for representative measurement lay midway on a straight line 

 between an electrode in the upstream ("A") row and either of the two 

 electrodes nearest it. in the center ("B") row. The average value, 

 expressed in volts per inch, of four such readings taken in the area 

 between two adjacent "A" row and the nearest three "D" row electrodes 

 was used as a representative figure of the "minimum voltage gradient" 

 produced in the particular array being tested. All voltage gradient 

 measurements described here were taken at midwater depth (halfway be- 

 tween surface and bottom) with the probe terminals in a horizontal 

 plane and oriented for maximum voltage reading at the specified point 

 of measurement. 



The voltage gradient probe is, in addition, an effective tool 

 for the detection of weak spots, i.e., areas of less than desired volt- 

 age gradient where escapement might result. When electrically shielded 

 traps are used in conjunction with an electrical weir, it is possible to 



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