Potential of an Induction- Coil at " make." 329 



The addition of series inductance to the primary circuit 

 exerts a minor influence on the secondary potential at make 

 in two other ways, viz. : — (1) Since P is diminished the 

 period 'Zirx/hoCzil — k 2 ), and with it the effective time of 

 action t x of the damping forces, is increased ; this tends to 

 reduce the maximum potential V2 m : (2) the ratio Ri/Lj, and 

 therefore also kj and S, may be altered ; e.lg., if Rj is kept 

 constant ^ and 8 are reduced ; this tends to increase V 2to . 

 In all ordinary circumstances these two effects are probably 

 much smaller than the first. 



The series inductance also increases the secondary potential 

 at the interruption of a given primary current, and increases 

 the spark-length of the coil for a given mean primary current. 

 These effects arise not only from the fact that the magnetic 

 energy at break is increased by increasing L 1? but also because 

 the coupling may be thus reduced to a certain point without 

 loss — and generally with gain — of efficiency of conversion of 

 primary magnetic into secondary electrostatic energy*. Thus 

 the process serves the double purpose of increasing the 

 potential at break and diminishing the potential at make. 



It is a well-known fact, first observed by Duddell f, that 

 inverse current through an X-ray tube also sometimes appears 

 at break. In this case it appears to be due to a kind of delayed 

 discharge, the discharge failing to pass in the first half- 

 oscillation after break, but passing in the second, or negative, 

 half-oscillation. As far as the writer has observed, this effect 

 can be prevented by sufficiently increasing the positive 

 potential at break : the higher this is the less likely is the 

 discharge to be delayed beyond it. Thus the use of series 

 inductance in the primary circuit tends to prevent the 

 occurrence of inverse current at both make and break. 



In some coils there is another way in which the primary 

 self-inductance can be changed, viz.: by connecting the 

 layers of the primary coil in series or in parallel or in other 

 ways. If the layers be regarded as all having the same 

 magnetic effect (for the same current), these changes of con- 

 nexion do not affect the coefficient of coupling, though the 

 mutual inductance and the primary self-inductance them- 

 selves are greatly altered. For example, if there are four 

 layers and the connexion is changed from series to parallel, 

 L 2 i and L ]2 are reduced to one quarter, L : to one sixteenth, 

 of their original values. Thus k 2 is unaltered while L^/L^ 

 is increased to four times its original value. If acting alone 

 this would, by (15), increase the secondary potential at make 



* Phil. Mag. Aug. 1915, pp. 233-235. 



t Journ. Roat. Soc. vol. iv. 17, p. 7 (1908). 



Phil. Mag. S. 6. Vol. 33. No. 196. April 1917. Z 



