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with a marble spool standard for which the Bureau of Standards had 

 furnished values. These coils represent several makers, and one, No. 7, 

 was made by the writer. It is known to be the practice of some makers 

 to wind the coils with only approximate measurements and data, then to 

 standardize them against a known value and subsequently to adjust cer- 

 tain factors in the data so that the calculated and measured values agree. 

 It has long the writer's belief that, except as a brief laboratory 

 exercise, to show the student how a standard mutual inductance may be 

 realized, the coaxial solenoids should be replaced by calibrated standards, 

 wound preferably on white marble spools thoroughly varnished and baked 

 hard. These when calibrated at the Bureau of Standards or elsewhere 

 are very permanent, convenient and relialtle. 



Purdue University. 

 Lafayette, Ind, 



