Lord Rayleigh on the Induction- Coll. 591 



Returning to the theoretical aspect of the question, and 

 inquiring whether there is any reason for expecting a con- 

 denser to give an advantage as compared with an absolutely 

 sudden cessation of the primary current, it is difficult to see 

 ground for other than a negative answer. In the case of no 

 magnetic leakage, somewhat closely approached, one would 

 suppose, in practice, an instantaneous abolition of the primary 

 current throws the whole available energy into the secondary 

 circuit, and thus, doing all that is possible, allows no room for 

 an improvement. Under such conditions a condenser can 

 only do harm. 



In the opposite extreme case of but a relatively small 

 mutual induction between primary and secondary, it is indeed 

 conceivable that the action of a condenser maybe advantageous. 

 The two currents would then be comparatively independent 

 and. ir the resistances were low, they might execute numerous 

 oscillations. If the primary current were simply stopped, 

 the effect in the secondary would be small ; whereas, espe- 

 cially if there were synchronism, the vibrations of the primary 

 current rendered possible by the condenser might cause an 

 accumulation of effect in the secondary. The case would be 

 that of ,; intermittent vibrations " *, such as may occur when 

 a large tuning-fork is clamped in a vice. A vibration, started 

 by a blow, in one prong gradually transfers itself to the other. 

 But it is difficult to believe that anything of this sort occurs 

 in an induction-coil as actually used. 



I do not know how far the theoretical arguments here 

 advanced will convince the reader that the use of a condenser 

 in the primary circuit should offer no advantage as compared 

 with a sufficiently sodden simple break ; but I may confess 

 that I should have hesitated to put them forward had I not 

 obtained experimental confirmation of them. My earlier 

 attempts in this direction were unsuccessful. A quick break 

 was constructed in which a spring, bearing upwards against a 

 stop, could be knocked away by a blow with a staff', or by a 

 falling weight. Although the contacts were of platinum, but 

 little advantage was gained in comparison with the ordinary 

 platinum break of the coil. Thus in one set of experiments, 

 where the coil was excited by a -ingle Grove cell, a break 

 made quickly by hand gave a spark about 8 mm. long. The 

 use of a weight, hung by a cotton thread, and falling 

 through about 1 2 feet when the thread was burned, increased 

 the length only to 8J mm. This was without a condenser. 

 When the condenser was applied, the spark-length was 



* " Theoy of Sound,' vol. i. § 114. 



