Potential of an Induction- Coil at "make." 327 



long. With a battery of 98 volts and the same resistance in the 

 primary circuit the spark-length at make was 4*5 mm. These 

 sparks passed between two ball electrodes each 2 cm. in 

 diameter, and therefore represent potentials of about 3500 and 

 16000 volts respectively. When the primary resistance was' 

 increased to 50 ohms the spark-length at make (E = 98 volts) 

 only fell to 37 mm. 



Thus the potential at make increases rapidly with the 

 battery E.M.F., but only varies slightly with the primary 

 current. On the other hand, the potential at break depends 

 oreatly upon the primary current, being proportional to the 

 value of this current at the moment of interruption. Conse- 

 quently it is desirable in working an induction-coil, with a 

 view to reducing the secondary potential and the inverse 

 current at make, to employ the smallest battery that is 

 sufficient to produce the required potential at break. Fre- 

 quently, however, an induction-coil is for reasons of con- 

 venience worked from a source at constant potential, such as 

 the 100-volt or 200- volt supply-mains, through a series 

 regulating resistance, in which case there is no possibility 

 of adjusting E. In any case, whatever be the source of 

 supply, it is desirable to consider what other means there 

 may be for reducing the potential at make. 



The expression (15) for the potential at make contains the 

 factor L 21 , and the potential may accordingly be reduced by 

 diminishing the mutual inductance of the primary and 

 secondary coils. This may be effected without causing any 

 alteration in L x by withdrawing the primary and the core 

 axially from their symmetrical position in the secondary. 

 In addition to reducing L 21 such a displacement has tho 

 effect of diminishing L 2 C 2 , and consequently also the period 

 of the oscillation. Thus the time t x during which the 

 damping forces act before the maximum make-potential is 

 attained is diminished, and this tends to increase the 

 maximum. On the whole, however, the withdrawal of the 

 primary is found to have the effect of reducing the spark- 

 length at make. On one occasion, for example, the 18-inch 

 coil with the primary in the position of maximum mutual 

 inductance gave a make-spark 4*4 mm. long (E = 98 volts, 

 R) = 12 ohms). When the primary was drawn out 30 cm. 

 from this position the spark-length at make was only 1*5 mm. 

 It might be thought that the displacement of the primary 

 coil would, owing to the decrease of mutual inductance which 

 it entails, cause a diminution of the secondary spark-length 

 at break, but this is not necessarily the case. By drawing 

 out the primary the system may be brought into one of the 



