330 Professor Oliver Lodge [June 1, 



express it as a modification of the Volta contact effect * with 

 illumination. 



Return now to the Hertz vibrator, or Leyclen jar with its coatings 

 well separated, so that we can get into its electric as well as its magnetic 

 field. Here is a great one giving waves 30 metres long, radiating 

 while it lasts with an activity of 100 H.P., and making ten million 

 complete electric vibrations per second (Fig. 10). 



Its great radiating power damps it down very rapidly, so that it 

 does not make above two or three swings ; but nevertheless, each 

 time it is excited, sparks can be drawn from most of the reasonably 

 elongated conductors in this theatre. 



A suitably situated gas-leak can be ignited by these induced 

 sparks. An Abel's fuse connecting the water pipes with the gas 



Fig. 10. 



^^- 



Large Hertz Oscillator on reduced scale, -^ inch to a foot. 



pipes will blow off; vacuum tubes connected to nothing will glow 

 (this fact has been familiar to all who have worked with Hertz waves 

 since 1889) ; electric leads, if anywhere near each other, as they are 

 in some incandescent lamp holders, may spark across to each other, 

 thus striking an arc and blowing their fuses. This blowing of fuses 

 by electric radiation frequently harvpened at Liverpool till the 

 suspensions of the theatre lamps were altered. 



The striking of an arc by the little reverberating sj)arks between 

 two lamp-carbons connected with the 100-volt mains I incidentally 



after a few fine days. I have now tried these experiments on such geological 

 fragments as were handy, and find that many of them discharge negative elec- 

 tricity under the action of a naked arc, especially from the side of the specimens 

 which was somewhat dusty, but that when wet they discharge much less rapidly, 

 and when positively charged hardly at all. Ice and garden soil discharge negative 

 electrification, too, under ultra-violet illumination, but not so quickly as limestone, 

 mica schist, ferruginous quartz, clay and some other specimens. Granite barely 

 acts : it seems to insulate too well. The ice and soil were tried in their usual 

 moist condition, but, when thoroughly dry, soil discharges quite rapidly. No 

 rock tested was found to discharge as quickly as does a surface of perfectly bright 

 metal, such as iron, but many discharged much more quickly than ordinary 

 dull iron, and rather more quickly than when the bright iron surface was thinly 

 oiled or wetted with water. To-day (June 5) I find that the leaves of Geranium 

 discharge positive electrification five times as quickly as negative, under the 

 action of an arc-light, and that glass cuts the effect off while quartz transmits it. 



[Added later. Arrhenius has had the same notion about atmospheric elec- 

 tricity ; and Elster and Geitel have made elaborate and careful experiments on 

 the subject. Wied. Ann. vols, xxxix., xl., xli., &c ] 



* See Brit. Assoc. Report, 1S84, pp. 502-519, or Phil. Mag., vol. xix. 

 pp. 267-352. 



