218 PROCEEDINGS OF THE AMERICAN ACADEMY 



XV. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF 

 HARVARD COLLEGE. 



No. VIII. — ON THE INDUCTION SPARK PRODUCED IN BREAKING 

 A GALVANIC CIRCUIT BETWEEN THE POLES OF A 

 MAGNET. 



By B. O. Peirce, Jr. 



Presented, Feb. 9, 1875. 



Becquerel, in the " Journal de Physique," IV. 206, states that 

 the spark obtained by breaking the current, which circulates through 

 an electro-magnet between its poles, is entirely a mechanical effect. 



To test the accuracy of M. Becquerel's I'esult, I placed in the same 

 circuit four Grove cells, an electro-magnet of the form used in experi- 

 ments upon Diamagnetism, and a coil of coarse wire wound around a 

 bundle of iron wires on a core. Over this coil was slipped an induc- 

 tion coil of 6000 ohms resistance. 



I first experimented Upon the spark given by the induction coil 

 when the primary circuit was broken between the poles of the electro- 

 magnet, and then outside of them. 



The number of cells in the circuit was varied, and its resistance 

 was several times changed, in order to vary the circumstances as much 

 as possible ; and, at each change, a series of observations were taken to 

 see whether the spark ft'om the induction coil had any greater power 

 of overcoming resistance when the primary circuit was broken between 

 the poles of the magnet. 



A long series of observations on sparks, which were of all lengths 

 from 2 1 to 15 mms., justifies the statement that the distance over 

 which the spark of the induction coil would regularly leap was more 

 than doubled when the circuit was broken between the poles. The 

 absolute length of spark obtained depended, of course, on the electro- 

 motive force in the primary circuit being apparently within certain 

 limits directly proportional to it. In almost all cases, the length of 



