1 78 On the Extra Currents through the Electric Spark. 



arose in the electric spark, only three-quarters traversed the 

 galvanometer. If there is a bridge between i and k, the re- 

 sistance of which is small as compared with the resistance in the 

 spark and in the rheostat m, the greatest part of the induction 

 produced in the galvanometer passes through the bridge; and as 

 they are equal in quantity and opposite in direction, their action 

 on the magnetic needle is eliminated. But if the bridge is re- 

 moved, the induction-currents act upon the magnetic needle. 



If, now, this action is in the opposite direction to that which is 

 caused by the spark, the deflection on inserting the bridge must 

 be more than three-fourths of that which ensues when the bridge 

 is removed.' 



The following experiments show that the first deflection is even 

 considerably greater than the latter. 



Experiment III. — The bridge inserted between the points i 

 and k. There were thus obtained the following deflections when 

 the machine was at work : — 



Divisions. 

 24-0 

 26-0 

 25-5 



Mean . . 25*2 



Without the bridge the deflections were 



13-3 

 123 

 13-3 



12-8 



Mean . . 129 



The bridge was again introduced, upon which the deflections 

 became 



26-2 



232 



23-7 



20-7 



Mean . . 235 



If the mean be taken of the first and third means, the number 

 24*35 is obtained, which is double as much as when the bridge 

 was removed. A few other experiments, which it is superfluous 

 to publish here, showed that the amount of diminution in the 

 deflection of the magnetic needle which the induction-currents 

 produce was, by far, not proportional to the number of turns of 

 the induction- spiral, but increased much more slowly. 



