242 



PROCEEDINGS OP THE AMERICAN ACADEMY 



The corresponding inverse electromotive forces are 11.7 volts and 

 5.6 volts respectively. Both of these are much below the value 

 already found for the whistling arc under normal conditions. But 

 we also find here, for the first time, a great difference between the 

 inverse electromotive force with different currents. As with the nor- 

 mal arc, the higher value occurs with the lesser current. With the 

 current of 7 amperes the conductive resistance is lower than with the 

 normal arc, but with 8 amperes it is higher. Owing to the low inverse 

 electromotive force, the total resistance is considerably less with both 

 strengths of current. 



It was our intention to observe also the effect of cooling the negative 

 carbon, but this was prevented by an accident to the apparatus. There 

 is no reason to suppose, in the light of Edlund's experiments already 

 cited, that this would have made any change in the nature of the results. 



This fall of inverse electromotive force with lowered temperature 

 of the arc may be the explanation of the fall which occurs with the 

 inverted arc, as the increased convection when the hot positive crater 

 is turned upward may be sufficient to lower the temperature by a 

 slight amount. 



The following tables give the data from which our equations were 

 determined. 



TABLE XXIII. 

 Positive Carbon Heated. 

 Current ^= 7 amperes. 



TABLE XXIV. 

 Positive Carbon Cooled. 



Current == 7 amperes. 



For observations on the arc formed under diminished pressure and 

 in different gases, a metallic receiver was constructed in which the 

 rod carrying the upper carbon passed through the closed top of the 

 receiver, and was movable by means of a micrometer screw. A win- 



