208 



LORD RAYLEIGH. 



and thus, doing ail that is possible, allows no room for an improvement. 

 Under such conditions a condenser can only do harui. 



Tn tlie opposite extrême case of but a relatively small mutual induc- 

 tion between primary and secondary, it is indeed conceivable that the 

 action of a condenser may be aclvantageous. The two currents would 

 then be comparatively independent and, if the résistances were low, 

 they might exécute numéro us oscillations. If the primary current were 

 simply stopped, the effect in the secondary would be small; whereas, 

 especially 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. À 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 sumciently sudden simple 

 break; but I may confess that I should have hesitated to put them for- 

 ward had I not obtained expérimental confirmation of them. My earlier 

 attempts in this direction were unsuccessful. A quick break was con- 

 structed in which a spring, bearing upwards against a stop, could be 

 knocked aivay by a blow with a staff, or by a falling weight. Àlthough 

 the contacts were of platinuin, but little advantage was gained in com- 

 parison with the ordinary platinum break of the coil. Thus in one set 

 of experiments, where the coil was excitée! by a single 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 12 

 feet when the thread was burned, increased the length only to 8| mm. 

 This was witliout a condenser. When the condenser was applied, the 

 spark-length was 14 mm , and it made no perceptible différence whether 

 or not the falling weight was employée!. Considering that the velocity 

 of the weight at impact must have been about 30 feet per second and 

 that its mass was large comparée! with that of the spring, thèse results 

 were far from promising. With a stronger primary current the aelvan- 



l ) „Theory of Sound," vol. I. § 114. 



