Induction Spark between Magnetic Poles. 461 



should retrace our steps from this practical and partly realized 

 ideal, and revert to those lesser paths in which the science of 

 physics should be developed, must be discussed in a future 

 communication. 



LIV. On the Induction Spark produced in breaking a Galvanic 

 Circuit between the Poles of a Magnet. By B. 0. Peirce, 

 Jun* 



BECQUEREL, in the Journal de Physique, vol. iv. p. 206, 

 states that the spark obtained by breaking the current 

 which circulates through an electromagnet, between its poles, 

 is entirely a mechanical effect. 



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

 the same circuit four Grove cells, an electromagnet of the form 

 used in experiments upon diamagnetism, and a coil of coarse 

 wire wound round a bundle of iron wires on a core. Over 

 this coil was slipped an induction-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 electromagnet, and then outside of them. 



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

 ance was several times changed, in order to vary the circum- 

 stances as much as possible ; and at each change a series of 

 observations were taken to see whether the spark from the in- 

 duction-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 J to 15 millims., 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 ob- 

 tained depended, of course, on the electromotive force in the 

 primary circuit being apparently, within certain limits, directly 

 proportional to it. In almost all cases the length of spark was 

 increased to three or four times ; and in no one set of observa- 

 tions was the spark less than doubled in length. These experi- 

 ments were repeated several times on different days. 



I found great difficulty in breaking the primary circuit uni- 

 formly ; and it was only after a long time that my results 

 became regular. 



I tried breaking from the surface of mercury ; but although 

 the same relative effects were attained, the actual effect was far 



* Communicated by the Author, having been read at the American 

 Academy of Arts and Sciences, February 9, 1876. 



