280 



National Resources Planning Board 



(oUifjiic CJuiegrali 



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Some simple engineering prob- 

 lems require advanced mathe- 

 matics in their solution. This 

 is true, for example, in the 

 computation of the magnetic 

 field outside the spiral grid of 

 a vacuum tube, a problem of 

 interest to Bell Telephone Lab- 

 oratories. If the grid is closely 





^ 



; ^ coiled, the current can be 



treated as a continuous cylin- 



. drical sheet, of radius a. Then 



/ / / ' the component of the magnetic 



1 field parallel to the axis of the 



/ ' ' grid at a distance r from the 



f A , axis is given by the above func- 



/ ' tion of two Elliptic Integrals 



whose "modulus" is k=a/r. 



Figure S7 



