146 Prof. E. Taylor Jones on the Potential 



essentially similar to the ordinary induction-coil, it differs 

 from it in the following respects : — 



(1) One terminal of the condenser, one of the contact-pieces 

 of I, one end of the primary winding and one side of the 

 secondary spark-gap G, are all connected together. This is 

 effected by connecting each of these four points with "earth," 

 i. e. with the frame of the machine. 



(2) The secondary wire is connected at one end with the 

 primary, from this point of junction J being led the connexion 

 with the contact-breaker and the condenser. The secondary 

 coil is thus " earthed " through the primary. 



(3) Instead of a battery, the rotation of the armature 

 between the poles of the permanent magnet serves, while the 

 contacts I are closed, to establish the primary current. At a 

 certain point in the revolution, at or near which the primary 

 current would have its maximum value if the contacts 

 remained closed, the contact at I is broken ; thereupon the 

 high-tension effect is produced in the secondary coil. The 

 rotation has also the effect of inducing electromotive forces, 

 in both primary and secondary coils, which are maintained 

 after the contacts are separated. 



It is generally admitted that the secondary potential 

 causing the spark arises mainly from the interruption of the 

 primary current, and is only contributed to in small measure 

 by the induced E.M.F. due to rotation. Thus in any given 

 machine the secondary potential depends mainly upon the 

 current t in the primary coil at the moment of break, and is 

 in fact, as in the induction-coil, approximately proportional 

 to this current. 



With regard to the value of i Q , a graphical method has been 

 given by A. P. Young * for determining this current when 

 the open-circuit primary voltage curve, the resistance of 

 the primary circuit, and the primary self-inductance in 

 various positions of the armature are known. The value 

 of i determined by the graphical method is said to agree 

 substantially with that shown by an oscillograph. We may 

 therefore conclude that the manner of growth of the primary 

 current after "make " is well understood, and that methods 

 are available for determining with sufficient accuracy the 

 value of this current at the moment of " break." 



The present communication is mainly concerned with what 

 goes on after the contacts are separated, and especially with 

 the manner in which the secondary potential rises and in which 

 its value depends upon the properties of the circuits. The 

 constants and coefficients upon which the secondary potential 

 depends may be enumerated as follows : — 



* < The Electrician,' Sept. 14, 1917, p. 923. 



