OP ARTS AND SCIENCES. 



237 



The curves for the inverted arc were constructed, as already ex- 

 plained for the upright arc. These were found, as with the latter, 

 to consist of two distinct straight lines, corresponding to the silent and 

 the whistling arcs respectively. The following equations (Tables XVI., 

 XVII.) represent the results obtained. Equations 10 and 12 were 

 found by taking the extreme readings as described, the current being 

 steady at 5.1 amperes. Equation 11 was obtained from another series 

 of experiments, using the same strength of current, but taking the 

 mean reading of the voltmeter at each observation. Equations 14 

 and 15 were constructed from the extreme readings. The current 

 varied slightly with these last, the average value being 9.6 amperes 

 for the silent arc and 10.2 amperes for the whistling arc. The inverse 

 electromotive force is given in the last column of the table. 



TABLE XVI. — Silent Arc. 



TABLE XVIL — Whistling Arc. 



It will be seen from the figures in the last column that the fluctua- 

 tions of the voltmeter to which we have referred were, for the most 

 part, due to variations in the conductive resistance of the arc, the 

 inverse electromotive force being almost constant for the same cur- 

 rent, notwithstanding the variations occurring in the total resistance. 

 With the whistling arc the inverse electromotive force is decidedly 



