ON THE INDTICTION-COIL. 



211 



suggest improvements in the design of coils. The first requirement is 

 evidently the existence of sufficient energy at break, and this implies a 

 considérable mass of iron, well magnetized, and not forming a circuit 

 too nearly closed. The full utilization of this energy is impeded by 

 want of suddenness in the break, by eddy -currents in the iron, and (in 

 respect of spark-length) by capacity in the secondary. It is to be pre- 

 sumed that in a well-designed coil thèse impediments should operate 

 somewhat equally. It would be useless to subdivide the iron, or to 

 reduce the secondary capacity, below certain limits, unless at the same 

 time the break could be made more suclden. It would not be surprising 

 if it were found that the tentative efforts of skilful instruinent-makers 

 have already led to a suitable compromise, at least in the case of coils 

 of moderate size. The design of larger instruments may leave more to 

 be accomplished. 



14* 



