﻿Effective Adjustment of an Induction-coil. 5 



one state of unit efficiency to another, e. (j. from the 

 second to the first adjustment of Table L. for it is only 

 in these states that equations (3 a), (3 b) hold. If, on the 

 other hand, k lies between two of the unit-efficiency values, 

 the spark-length for a given current may be increased 

 by withdrawing the primary coil and core to a suitable 

 distance along the axis of the secondary. For example, 

 the coil with which I have experimented has a coupling 

 coefficient of '876, and the longest spark occurs when the 

 primary capacity is about *06 microfarad *. A marked 

 increase is, however, produced in the spark-length by with- 

 drawing the primary coil and core to a distance of 36*5 cm. 

 from its nearly symmetrical position in the secondary, and 

 increasing the capacity to *15 microfarad. 



We may conclude that any induction-coil in which the 

 coupling is greater than *756 can be improved in spark- 

 length by connecting external series inductance in the 

 primary circuit so as to reduce the coupling to this value. 

 An induction-coil should not be constructed so that the 

 coupling is less than *756, for there is no convenient way 

 of increasing the coupling to this value. Nor should the 

 coupling coefficient of the primary and secondary coils have 

 precisely this value, though if no external coils are used 

 this is the best arrangement. Better effects are obtained by 

 coupling the coils more closely than this, and adding series 

 inductance to the primary circuit so as to reduce h to this 

 value. The ratio LJC2 is thereby increased, and it is upon 

 this quantity, when the adjustments (1) and (2) are effected, 

 that the maximum secondary potential for a given primary 

 current chiefly depends. 



In fig. 1 (A) are shown curves representing the two 

 potential waves in the primary circuit of an induction-coil 

 in which the above adjustment has been effected, and in 

 which the damping of the oscillations is negligible. The 

 amplitude of each wave is taken as 5000 volts, and the 

 periods are *002I sec. and *0008 sec. These values are chosen 

 ■so as to correspond approximately (for i = 10 amperes) with 

 those found in an actual case, as described below. Fig. 1 (B) 

 shows the result of superposing the two curves of (A). The 

 equation to curve (B), the ordinate of which represents the 

 potential of the primary condenser t seconds after the inter- 

 ruption, is 



Vi= -5000 sin 150,000 *-5000 sin 450,000 1, 

 the angles being given in degrees. 



* It was explained in the previous paper (I, e, pp. 584. 585) how the 

 cnost effective primary capacity in any such case can be calculated. 



