Capacity for Induction-coils and Tesla Coils. 235 



The curve CBA shows how the maximum secondary potential 

 for a given primary current may be increased by adding series 

 inductance to the primary circuit, in the case of an ideal 

 induction-coil with negligible resistances. In practice the 

 curve is, owing to the damping effect of the resistances, 

 considerably below the curve CBA of fig. 3. It may some- 

 times bend downwards near the point corresponding to a, as 

 I found in some experiments described recently*. I was then 

 under the impression that this is theoretically correct in all 

 cases, but curve A shows that this view is erroneous ; and on 

 repeating the experiments, using for the series inductance 

 only air-core coils of low resistance, I found that the spark- 

 length continued to increase when the self-inductance was 

 increased beyond the value corresponding to the point af. 

 If suitable coils are used for the series inductance, and if the 

 interruptions are good, the experimental results therefore 

 conform in this respect with the theory as represented by 

 the curve in fig. 3. The fact is that as the primary self- 

 inductance is increased more energy is being put into 

 the system, and, while the efficiency falls off from the 

 point a, the maximum secondary potential continues to 

 increase. This increase becomes, however, less and less 

 rapid as L x is increased, and the facts that it is accompanied 

 by loss of efficiency, that under less favourable circumstances 

 it is converted into a diminution, and that a large increase 

 of self-inductance gives rise to troublesome sparking at the 

 interruptor, indicate that it is inadvisable to continue the 

 process beyond the above-mentioned point. 



In the Annalen der Physik, vol. v. p. 837 (1901), an induc- 

 tion-coil is described by Klingelfuss in which the primary 

 and secondary coils are wound on a nearly closed magnetic 

 circuit. The instrument is said to give a very long spark 

 with a comparatively small number of turns in the secondary 

 coil. The advantage of such an arrangement appears to 

 arise from the low electrical resistance of the secondary coil 

 and the low magnetic reluctance of the core. Since L 2 i is 

 approximately proportional to the number of secondary turns, 

 and L 2 to its square, the factor L 2 i/ s/h 2 of equation (9) is 

 not changed by a reduction of the number of turns, but the 

 lowering of secondary resistance, due both to the reduction 

 of the length of wire and to the use of thicker wire for the 

 secondary coil, diminishes the damping of the oscillations 

 and thus tends to allow a higher secondary potential to be 



* Phil. Mag. January 1915, p. 7. 



t This effect was not found when coils with iron-wire or sheet-iron 

 cores were used. Probably Maxwell's coil of maximum self-inductance 

 for given length and diameter of wire is the best form for the purpose. 



