2U 



PROCEEDINGS OF THE AMERICAN ACADEMY 



irrespective of the length of the arc (witliin the limits of whistling), 

 but it also has, like that of the silent arc, an approximately definite 

 value for all currents. The mean of the results with the silent arc 

 given in the table is about 15 volts. There is, also, notwithstanding 

 some anomalous results, the same manifest diminution with increase 

 of current that we observe with the silent arc. It will further be 

 noticed, that the constant, b, diminishes as the current increases, but 

 that its value is several times as great for the whistling as for the 

 silent arc. 



For both the silent and whistling arcs, the conductive resistance, 

 bl, diminishes as the current is increased, at first rapidly, afterwards 

 more slowly. 



The equations verify previous observations, in that the passage 

 from higher to lower inverse electromotive force is a sudden one. It 

 takes place with a greater separation of the carbons, and less equiva- 

 lent resistance, as the current is stronger, a result agreeing with that 

 reached by White from direct measurements.* The following figures 

 (Table X.), taken from the curves as plotted, illustrate this. The 

 first column gives the approximate current, the second the equivalent 

 resistance, the third the separation of the carbons at which the change 

 would take place, which last is, of course, the abscissa of the point of 

 intersection of the two straight lines representing the two varieties 

 of arc. 



TABLE X. 



These values would doubtlessly vary considerably with the nature 

 of the carbons used for the electrodes. 



It was also noticed with the stronger currents, that, on lengthening 

 the arc, the whistle ceased slightly before the higher inverse electro- 

 motive force was reached. This appeared to be somewhat influenced 

 by the quality of the carbons. In all cases, however, the line of 

 demarcation between the silent and the whistling arc was very defi- 



* Electrician, 1884, vol. xiv. p. 56. 



