1889.] on the Discharge of a Leyden Jar. 421 



well as Henry's well repay perusal, though, of course, they belong 

 to the infancy of the subject. 



He notes the striking fact that the bursting of a jar is an extra 

 occurrence, it does not replace the ordinary discharge in the proper 

 place, it accompanies it ; and we now know that it is precipitated by 

 it, that the spark occurring properly between the knobs sets up such 

 violent surgings that the jar is far more violently strained than by 

 the static charge or mere difference of potentials between its coatings ; 

 and if the surgings are at all even roughly properly timed, the jar 

 is bound to either overflow or burst. 



Hence a jar should always be made without a lid, and with a lip 

 protruding a carefully considered distance above its coatings : not 

 so far as to fail to act as a safety valve, but far enough to prevent 

 overflow under ordinary and easy circumstances. 



And now we come to what is after all the main subject of my 

 discourse this evening, viz. the optical and audible demonstration of 

 the oscillations occurring in the Leyden jar spark. Such a demon- 

 stration has, so far as I know, never before been attempted, but if 

 nothing goes wToug we shall easily accomplish it. 



And first I will do it audibly. To this end the oscillations must 

 be brought down from their extraordinary frequency of a million or 

 hundred thousand a second to a rate within the limits of human 

 audition. One does it exactly as in the" case of the spring — one first 

 increases the flexibility and then one loads it. [Spark from battery 

 of jars and varying sound of same.] 



Using the largest battery of jars at our disposal, I take the spark 

 between these two knobs — not a Lmg spark, i inch will be quite 

 sufficient. Notwithstanding the great capacity, the rate of vibration 

 is still far above the limit of audibility, and nothing but the 

 customary crack is heard. I next add inertia to the circuit by 

 including a great coil of wire, and at once the spark changes 

 character, becoming very shrill but an unmistakable whistle, of a 

 quality approximating to the cry of a bat. Add another coil, and 

 down comes the pace once more, to something like 5000 per second, 

 or about the highest note of a piano. Again and again I load the 

 circuit with magnetisability, and at last the spark has only 600 vibra- 

 tions a second, giving the octave, or perhaps the double octave, above 

 the middle C. 



One sees clearly why one gets a musical note : the noise of the 

 spark is due to a sudden heating of the air ; now if the heat is oscil- 

 latory, the sound will be oscillatory too, but both will be an octave 

 above the electric oscillation, if I may so express it, because two heat- 

 pulses will accompany every complete electric vibration, the heat 

 production being independent of direction of current. 



Having thus got the frequency of oscillation down to so manage- 

 able a value, the optical analysis of it presents no difficulty : a simple 

 looking-glass waggled in the hand will suffice to spread out the spark 



