142 



Mr. W. L. Upson : Observations 



than 15 amperes. At 7 or 8 amperes the arc would hardly 

 form at all. 



^Copper-copper in air would not start at all. It seems 

 probable that if sufficient heat were concentrated at the ends 

 of the poles, the arc could be maintained. 



Interrupted Arcs. 



It was desired to compare the lengths of time that various 

 arcs could be interrupted without losing the power of re- 

 starting themselves when the voltage was again applied to 

 their terminals. For this purpose, apparatus was devised in 

 accordance with a suggestion by Dr. Fleming as follows : — 



A small electromagnet was fixed so that its armature 

 would have a free fall when the current was switched off. 

 When the armature was held by the magnet, two springs 

 bore lightly against its sides and, they being in the main 

 arc circuit, the current of the arc passed through the 

 armature. "When the armature was allowed to fall, it was 

 guided into another set of spring-contacts in parallel with 

 the first set, thus completing the circuit, which in falling, it 

 had opened. 



The length of fall could be measured accurately, and the 



time was calculated from the formula £ = ^~— , where d is 

 the fall in feet. 



It was especially desired to compare the carbon-carbon arc 

 in air and the copper-carbon arc in hydrogen. With the 

 currents used, the spark on opening the circuit was in all 

 cases so small as to be of inappreciable length, and therefore 

 its effect on the value of t could be neglected. The following 

 table is for solid carbons in air, with arc-length of O05 in. 



Volts. 



Amperes. 



Fall in feet. 



Time 



(sec). 



Watts. 



45 



4-5 



0-3 



0137 



2025 



49 



3 



0146 



0-095 



147 



43 



6 



0-541 



0-184 



258 



41 



7-5 



0-833 



0-228 



307-5 



40 



9 



1-44 



0-3 



360 



Plotting either amperes or watts against time gives nearly 

 a straight line (see fig. 12, PI. VII.). 



Both lines, continued downward, cut through the origin. 



