658 Length of the Break-Spark in an Inductive Circuit. 

 Fig. 6. — Zinc and Lead. 



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Current in Amperes. 



16 



of shape, indicating that the spark-length varies with the 

 current in different ways for different materials so that the 

 material which gives the longest spark with a particular 

 current does not necessarily give the longest spark with 

 another current. Thus with a current of 15 amperes the 

 spark is longest with carbon poles, shortest with platinum 

 ones ; but, on reducing the current to 3 amperes, the easily 

 fusible metals bismuth and lead give the longest sparks. 



Perhaps the most interesting point brought out, however, is 

 in connexion with the behaviour of the different materials on 

 reversal of the current. With the metals iron, copper, tin, 

 nickel, zinc, and with carbon, reversing the current produces 

 no perceptible difference in the spark-length ; but with lead, 

 platinum, and aluminium poles the spark-length for the larger 

 currents is notably greater when the fixed pole is negative 

 than when it is positive. 



Owens College, Manchester. 



