1894.] 



on the Work of Hertz. 



335 



Fig. 1G. 



clown. Very slight electric surgings precipitate the discharge across 

 the gap, and the leaves diverge. I show this in a modified and 

 simple form. On the cap of an electroscope is placed a highly- 

 polished knob or rounded end connected to the sole, and just not 

 touching the cap, or rather just not touching a plate connected with 

 the cap, Fig. 16, the distance between knob and plate being almost 

 infinitesimal, such a distance as is appreciated in spherometry. Such 

 an electroscope overflows suddenly and completely with any gentle 

 rise of potential. Bring excited glass near it, the leaves diverge 

 gradually and then suddenly collapse, because the air space snaps ; 

 remove the glass and they rediverge with negative electricity; the 

 knob above the cap being then 

 charged positively and to the 

 verge of sparking. In this con- 

 dition any electrical waves, col- 

 lected if weak by a foot or so of 

 wire projecting from the cap, will 

 discharge the electroscope by ex- 

 citing surgings in the wire, and so 

 breaking down the air-gap. The 

 chief interest about this experi- 

 ment seems to me the extremely 

 definite dielectric strength of so 

 infinitesimal an air space. More- 

 over, it is a detector for Hertz 

 waves that might have been used 

 last century ; it might have been 

 used by Benjamin Franklin. 



For to excite them no coil or 

 anything complicated is neces- 

 sary; it is sufficient to flick a 

 metal sphere or cylinder with a 

 silk handkerchief and then dis- 

 charge it with a well-polished 

 knob. If it is not well polished 

 the discharge is comparatively 



gradual, and the vibrations are weak ; the more polished are the sides 

 of an air-gap, the more sudden is the collapse and the more vigorous 

 the consequent radiation, especially the radiation of high frequency, 

 the higher harmonics of the disturbance. 



For delicate experiments it is sometimes well to repolish the 

 knobs every hour or so. For metrical experiments it is often better 

 to let the knobs get into a less efficient but more permanent state. 

 This is true of all senders or radiators. For the generation of the, 

 so to speak, "infra-red" Hertz waves any knobs will do, but to 

 generate the " ultra-violet " high polish is essential. 



Receivers or detectors, which for the present I temporarily call 

 microphonic, are liable to respond best to the more rapid vibrations. 



Air-gap for Electroscope. Natural size. 

 The bottom plate is connected to, and 

 represents, the cap of an electro- 

 scope ; the " knob " above it, men- 

 tioned in text, is the polished end of 

 the screw, whose terminal is connected 

 with the case of the instrument or 

 " earth." 



