320 



LESSOVS IN ELECTRICITY. 



FKI. &1 



tho fonriGr is now known n tho ehctrie 

 brush, and the latter rs the electric glow. 

 Both c*n be produced in unconfined air. 

 Tho glow is sometimes seen on the masts, 

 of clips, and it is mentioned by tho an- 

 ci-euta as appearing on the points of 

 kacea. It is called St. Ermo's or St. 

 Elmo's fire, ?:ftcr the .*ailoiV saint, Eras- 

 inu*, \*ho nuHered martyrdom at Gaeta, 

 at the beginning of the fourth century. 



The purple color of the diffused light 

 in attenuated air was noticed by Hauks- 

 boc. The color depends upon the resi- 

 due of attenuated gas, or vapor, through 

 which the discharge passes. If it be an 

 oxygen-residue the light is whitish, if it 

 be a hydrogen-residue the light is red, if 

 a nitrogen-residue the light is purple, 

 fj;act!y resembling that displayed at times 

 by vlro aurora borealis a color doubtless 

 due to tho discharge of electricity 

 through the attenuated nitrogen of the air. 



Electric light in vacuo is readily pro- 

 duced by the friction of an amalgamated 

 FH I -^cr against the outside of an exhausted 

 tube. The light is also produced by the 

 friction of mercury within a barometric 

 vacuum. The discharges through tube* 



many feet in length and exhausted by 

 an air-pump are very fine. The double 

 barometer tube of Cavendish also yields 

 a truly splendid bow of light, when a 

 strong electric discharge is sent through 

 it. For this experiment fig. 52 shows 

 the best arrangement. p is the prime 

 conductor of an electrical machine, i an 

 insulated metal ball, connected by a wire 

 with the mercury trough A. The 

 trough u is connected by a wire with the 

 earth, c and c' mark the height of the 

 mercurial columns. When the machine 

 is worked sparks pass from p to i, a 

 vivid bow of light at each passage stretch- 

 ing from o to c'. By causing i to ap- 

 proach P, the discharges become more 

 frequent, t>nt more feeble ; by augment- 

 ing the distance P i, the sparks become 

 rarer, but more strong. When very 

 tro.ng, a bow of dazzling brilliancy ac- 

 companies every spark.* 



Small tubes tor these experiments are 

 hd*t obtained from philosophical instru- 

 ment makers 



27. Lichlenbtry's Figures. 



Licht.cnberg deriasd a me-xis of rc- 

 vaaling tho condition of an electrified 

 surface by dusting it with powder. It'sd 

 lead, in passing through mr-slin, is p</i- 

 tivcly electrified ; flower of sulphur M 

 negatively electrified. Whisking a fox's 

 brush over a cake of resin, and drawing 

 over the surface the knob of a Lcyden j*r, 

 positively charged, tho re^.n is rendered 

 in part negafivc and in part positive. 

 Dusting the mixed powder over the sur- 

 face, the Rulphur arrange* itself over the 

 positive places, and the red lead ovc? 

 the negative places, a verv beautiful pat- 

 tern being the result. 



This experiment of LicMenberg's con- 

 stituted the germ of Chladni's important 

 acoustical researches. k Chladni's fig- 

 ures " were the direct offspring of 

 ** Lichtcnbcrg's figures." 



28. Surface Compared with Mass. 

 Distribution cf Electricity in Hollow 

 Conductors. 



Monnier proved that the charge of a 



* It is well ta have tho interval p i fit 

 s"me distance from the how, srvthr.t Ihu liirht 

 of the spark shall not impair thu efl(x:t of ih 

 discharge upon the eye. 



