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ELECTRICITY. — A tubular dedrodynamometer for heavy currei 

 P. G. Aorew. Bulletin Bureau of Standards. 8 : — 1912. 



The field " coil" of the instrument consists of two coaxial copper tu - 

 thus giving a circular magnetic field in the space between the tu" es. 

 direct current the distribution of the current is assumed to be uniform 

 over the cross section of the tubes, but on alternating current, as is well 

 known, the current is crowded toward the outside of the inside tube and 

 toward the inside of the outside tube, and the amount of this change of 

 distribution depends upon the frequency. But if we have axial sym- 

 metry the magnetic field at any point is independent of the current dis- 

 tribution. For there is no magnetic field between the tubes due to the 

 outside tube, and the field at any point due to the current in the inside 

 tube symmetrically distributed about the axis depends only on the dis- 

 tance of the point from the axis, and therefore the total effect is inde- 

 pendent of the distribution of current, so long as it is symmetrical with 

 respect to the axis. 



There are two moving coils astatically placed between the tubes, one 

 above and one below the inner tube, and suspended by a phosphor bronze 

 strip. The common axis of the tubes is placed in a horizontal position 

 in order to permit the use of a vertical suspension. While the distribu- 

 tion of the current is not the same on alternating as on direct current, 

 the magnetic field in the space between the tubes and hence the torque 

 is the same. 



With water passing thru the inner tube, the instrument will earn- 

 continuously 5000 amperes, and without water cooling. 1200 amperes. 

 The current-carrying capacity of the moving coils is 0.06 ampere. 



Difficulties were at first encountered, due to distortion of the inside 

 tube by differential thermal expansion, and by magnetic impurities in 

 the moving coil, but these were finally overcome. 



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