638 Mr. NiJcola Tesla [Feb. 4, 



The study of the properties of the high frequency currents 

 obtained from these machines is very interesting, as nearly every 

 experiment discloses something new. 



Two coils traversed by such a current attract or repel each other 

 with a force which, owing to the imperfection of our sense of touch, 

 seems continuous. 



An observation, scarcely foreseen, is that a piece of iron, sur- 

 rounded by a coil through which the current is passing appears to 

 be continuously magnetised. This apparent continuity might be 

 ascribed to the deficiency of the sense of touch, but there is evidence 

 that in currents of such high frequencies one of the impulses pre- 

 ponderates over the other. 



As might be expected, conductors traversed by such currents are 

 rapidly heated, owing to the increase of the resistance, and the heating 

 effects are relatively much greater in the iron. 



The hysteresis losses in iron are so great that an iron core, even 

 if finely subdivided, is heated in an incredibly short time. To give 

 an idea, an ordinary iron wire of 1/16 inch in diameter inserted 

 within a coil having 250 turns, with a current estimated to be five 

 amperes passing through the coil, becomes within two seconds' time 

 so hot as to scorch wood. Beyond a certain frequency, an iron core, 

 no matter how finely subdivided, exercises a dampening eifect, and it 

 was easy to find a point at which the impedance of a coil was not 

 affected by the presence of a core consisting of a bundle of very thin 

 well annealed and varnished iron wires. 



Experiments with a telephone, a conductor in a strong magnetic 

 field, or with a condenser or arc, seem to afford certain proof that 

 sounds far above the usually accepted limit of hearing would be 

 perceived if produced with sufficient power. 



The arc produced by these currents possesses several interesting 

 features. Usually it emits a note the pitch of which corresponds 

 to twice the frequency of the current, but if the frequency be 

 sufficiently high it becomes noiseless, the limit of audition being 

 determined principally by the linear dimensions of the arc. A 

 curious feature of the arc is its persistency, which is due partly to 

 the inability of the gaseous column to cool and increase considerably 

 in resistance, as in the case with low frequencies, and, partly to 

 the tendency of such a high frequency machine to maintain a constant 

 current. 



In connection with these machines the condenser affords a par- 

 ticularly interesting study. Striking effects are produced by proper 

 adjustments of capacity and self-induction. It is easy to raise the 

 electro-motive force of the machine to many times the original value 

 by simply adjusting the capacity of a condenser connected in the 

 induced circuit. If the condenser be at some distance from the 

 machine, the difference of potential on the terminals of the latter may 

 be only a small fraction of that on the condenser. 



But the most interesting experiences are made when the tension 



