230 



PROCEEDINGS OP THE AMERICAN ACADEMY 



TABLE I. — Silent Arc. 

 Length of Arc = ts'i in. 



TABLE XL — Whistling Arc. 

 Length of Arc = si in. 



Owing to the great variability of the arc, it is of course impossible 

 to avoid having a considerable average deviation with a series of suc- 

 cessive measurements of current and electromotive force ; but, as will 

 be seen, in spite of the necessarily large limits of variation, consistent 

 results are nevertheless obtainable. 



About 1200 measurements were taken, embracing 60 series of 10 

 readings with reversals, using lengths of arc of from jl-g in. to ^ in., 

 and currents varying from 3 to 10 amperes. Ordinarily we made a 

 series of measurements with increasing lengths of arc, keeping the cur- 

 rent as nearly constant as possible by means of resistance coils. The 

 variations in the current were rarely more than a few hundredths of 

 an ampere, and as the current was measured simultaneously with the 

 electromotive force, these small changes did no harm. The equivalent 

 resistance for each length of arc, with the various strengths of cur- 

 rent, was obtained from the data given by the measurements. The 

 following tables show the results reached. The first column gives the 



