LESSONS IN ELECTRICITY. 



821 



FIG. 44. 



rnable substances by the electric spark. 

 This was first effected by Ludolf, at the 

 opening of the Academy of Sciences by 

 Frederick the Great at Berlin, on the 

 23d of January, 1744. With a spark 

 from the sword of one of the court cav- 

 aliers present on the occasion, Ludolf ig- 

 nited sulphuric ether. 



Dr. \\atson also made numerous ex- 

 periments on the ignition of bodies by 

 the electric spark. He fired gunpowder 

 and discharged guns. Causing, more- 

 over, a spoon containing ether to be held 

 by an electrified person, he ignited the 

 ether by the finger of an unelectrified per- 

 son. He also noticed that the spark va- 

 ried in color when the substances between 

 which it passed varied. 



These, and numerous other experi- 

 ments may be made with a far simpler 

 " machine" than any hitherto described. 

 It was devised for your benefit by Mr. 

 Cottrell. In the electric machine, as we 

 have learned, the prime conductor is 

 flooded with positive electricity through 

 the discharge of the negative from the 

 points against the excited glass. Your 

 gloss tube and rubber may be similarly 

 turned to account. A strip of sheet- 

 brass or copper, p, fig. 44, is sewn on to 

 the edge of the silk pad, R, employed as 

 a rubber. Through apertures in the strip 

 about twenty pin-points are introduced, 

 and soldered to the metal. When the 

 tube is clasped by the rubber, the rnetal 

 strip and points quite encircle the tube. 



When a fine wire, ID, connects the strip 

 of metal with the knob of a Leyden jar, 

 by every downward stroke of the rubber 

 the glass tube is powerfully excited, and 



hotly following the exciting rubber is t^ 

 circle of points. From these, against ths 

 rod, negative electricity is discharged,, 

 the free positive electricity escaping along; 

 the wire to the jar, which is thus rapid- 

 ly charged. 



The ignition of gas is readily effected! 

 by Cottrell's rubber. Connecting thee 

 strip of metal, R, fig. 45, with an insulat- 

 ed metallic knob, B, placed within a* 

 quarter or an eighth of an inch of an 

 uninsulated argand burner connected with 

 the earth, at every downward stroke of" 

 the rubber a stream of sparks passes be- 

 tween the knob and burner. If gas bo* 

 turned on, it is immediately ignited by 

 the stream of sparks. Blowing out the- 

 fiame and repeating the experiment, every 

 stroke of the rubber infallibly ignites the- 

 as. 



Sulphuric ether, in a spoon which ha*. 

 been previously warmed, is thus ignited ;; 

 but the ether soon cools by evaporation ; 

 its vapor is diminished by the cold, and 

 it is then less easy to ignite. Bisulphide 

 of carbon may be substituted for the 

 ether, with the certainty that every stroke 

 of the rubber will set it ablaze. The 

 spark thus obtained also fires a mixture 

 of oxygen and hydrogen. The two gasei 



