422 Professor Oliver Lodge [March 8, 



into a serrated band, just as can be done with a singing or a sensitive 

 flame, a band too of very much the same appearance. 



Using an ordinary four-square rotating mirror driven electro- 

 magnetically at the rate of some two or three revolutions per second, 

 the band is at the lowest pitch seen to be quite coarsely serrated ; and 

 fine serrations can be seen with four revolutions per second in even 

 the shrill whistling sparks. 



The only difficulty in seeing these effects is to catch them at the 

 right moment. They are only visible for a minute fraction of a revo- 

 lution, though the band may appear drawn out to some length. The 

 further away the spark is from the mirror, the more drawn out it is, 

 but also the less chance there is of catching its image. 



With a single observer it is easy to arrange a contact maker on 

 the axle of the mirror which shall bring on the discharge at the 

 right place in the revolution, and the observer may then conveniently 

 watch for the image in a telescope or opera-glass, though at the lower 

 pitches nothing of the kind is necessary. 



But to show it to a large audience various plans can be adopted. 

 One is to arrange for several sparks instead of one ; another is to 

 multiply images of a single spark by suitably adjusted reflectors, which 

 if they are concave will give magnified images ; another is to use 

 several rotating mirrors ; and indeed I do use two, one adjusted so 

 as to suit the spectators in the gallery. 



But the best plan that has struck me is to combine an intermittent 

 and an oscillatory discharge. Have the circuit in two branches, one 

 of high resistance so as to give intermittences, the other of ordinary 

 resistance so as to be oscillatory, and let the mirror analyse every 

 constituent of the intermittent discharge into a serrated band. There 

 will thus be not one spark, but several successive sparks, close enough 

 together to sound almost like one, separate enough in the rotating 

 mirror to be visible on all sides at once, and each one analysed into its 

 component alternations. 



But to achieve this one must have great exciting power. In spite of 

 the power of this magnificent Wimshurst machine, it takes some 

 time to charge uj) our great Leyden battery, and it is tedious waiting 

 for each spark. A Wimshurst does admirably for a single observer, 

 but for a multitude one wants an instrument which shall charge the 

 battery not once only but many times over, with overflows between, 

 and all in the twinkling of an eye. 



To get this I must abandon my friend Mr. Wimshurst, and return 

 to Michael Faraday. In front of the table is a great induction coil ; 

 its secondary has the resistance needed to give an intermittent dis- 

 charge. The quantity it supplies at a single spark will fill our jars 

 to overflowing several times over. The discharge circuit and all its 

 circumstances shall remain unchanged. [Excite jars by coil.] 



Eunning over the gamut with this coil now used as our exciter 

 instead of the Wimshurst machine — everything else remaining 

 exactly as it was — you hear the sparks give the same notes as before. 



