Spark-Length of an Induction- Coil. 



141 



discharge-rods were utilized, and the secondary spark-length 

 was taken to be the distance between these poles when so 

 adjusted that the number of sparks amounted to one-half the 

 number of makes or breaks of the primary. 



Make-Sparks. — For a given current established in the 

 primary, the only thing that can affect the length of the 

 make-spark of an induction-coil is the magnitude of the 

 electromotive force employed to produce the current. An 

 increase in the primary electromotive force requires an 

 increase in the resistance to keep down the current to its 

 former value. This means a primary circuit with a smaller 

 time-constant, which, in turn, entails a more rapid rise of 

 current, a greater induced electromotive force, and conse- 

 quently a longer secondary spark. High primary electro- 

 motive force must therefore correspond to long secondary 

 spark at make ; in fact, as the numbers of Table I. and the 

 dotted curves in figs. 2 and 3 show, there exists a rough pro- 

 portionality between the two. 



Table I. — Make-Sparks. 

 Current made in primary = 2 - 5 amperes. 



Primary E.M.F. 

 in Volts. 



Secondary Spark-length 

 in millimetres. 



10 



20 

 30 

 38 

 50 

 70 

 80 

 95 



•3 

 10 

 2-0 

 2-li 

 4-1 

 6-5 

 7-6 

 90 



Break-Sparks without Condenser. — When, however, a given 

 primary current is interrupted by breaking the circuit, there 

 are several circumstances which unite in determinino- the 

 length of the secondary spark, viz., (1) the material of the 

 primary poles, (2) the suddenness with which the primary 

 poles are separated, and (3) the magnitude of the electro- 

 motive force used to produce the current. Exactly in what 

 way each of these exerts an influence may be gathered from 

 the results collected together in Table II., from which the 

 full-line curves of figs. 2 and 3 have been plotted. 



