174 PROCEEDINGS OF THE AMERICAN ACADEMY. 



simply cut out the " spark " gap, but the spark line could not be seen 

 when both currents were on. Now when both currents were on I broke 

 the arc circuit, and nothing at all could be seen in the spectroscope ; 

 neither the arc nor the spark lines remained, although the spark current 

 was still passing. After remaining at the eyepiece of the spectroscope 

 about one second I began to see traces of the spark lines, and then they 

 soon came out with their usual brightness, and the spark discharge which 

 had been silent during that second of darkness assumed its usual noisy 

 character. 



This experiment shows that the gases of the arc do not furnish enough 

 resistance to the passage of a high voltage alternating current to cause 

 the discharge to assume the character of a spark for a full second after 

 the breaking of the arc current. This seems to confirm the second 

 suggestion above, to the effect that the conductivity of the gases de- 

 creases so slowly on the breaking of the arc current in air as to give rise 

 to no very high voltage, and so accounts for the non-appearance of the 

 spark lines in the rotating arc in air. 



These two experiments throw an interesting light upon the nature of 

 the spark. The spark at the arc gap in these experiments seems to be 

 due to neither the current nor to the voltage, but to some kind of an im- 

 pulse furnished by the sudden rush of electricity across the auxiliary 

 " spark " gap. 



In the second experiment, above described, the spark lines do not all 

 seem to come out at the same time. I hope in the near future to be 

 able to arrange an automatic apparatus for making and breaking the 

 currents and an adjustable occul ting-screen which will enable one to 

 photograph the spectrum of the spark at definite intervals of time after 

 the arc current is broken. A series of these photographs will probably 

 furnish an interesting story of the development of the spark spectrum. 



Physical Laboratory, 

 Northwestern University. 



