Intelligence and Miscellaneous Articles. 317 



four different glasses so that they vibrated transversally ; they became 

 only negatively electrified. If the same bow was drawn backwards 

 and forwards lengthwise along the same part of the plate without 

 producing a tone, it became only positively electric. 



b. Copper plates of 4 and 7 centims. diameter were whipped 

 with white silk in various ways. If the stroke was delivered 

 nearly perpendicular to the plate, the latter became strongly posi- 

 tive ; if it was more nearly grazing, the plate became just as strongly 

 negative. "Whether the plate was fresh cleaned with hydrochloric 

 acid, or by longer exposure to the air had become tarnished, made 

 no difference, nor yet the size of the plate. Further, by lightly 

 rubbing its entire rim (best with a silk or woollen cloth in the 

 form of cone-cover), the plate is always rendered negative; by 

 hard rubbing with the same silk on the same place, always 

 positive. Coarse woollen cloth appeared to excite less powerfully. 

 Zinc gave the same result. 



c. With brass the collateral circumstances appeared to have an 

 influence. A square plate intended for the production of Chladni's 

 sound-figures behaved exactly like the copper plate. The shallow 

 brass scale of a table-balance, however, gave both electricities only 

 when struck with silk, and when its surface was well cleaned. 

 Lastly, an old pound-weight could not be excited in different ways 

 with silk ; this could only be accomplished with a violin-bow, ac- 

 cording as the bow was applied to its thick main part, or to its 

 thin neck. 



d. A small scale-pan of fine silver gave both electricities only 

 with silk, with wool it only became negatively electric. 



e. A hard-gum plate always became negative when slowly stroked 

 with a tightly folded linen cloth — when stroked quickly, in other- 

 wise like circumstances, positive. The surface of the hand produced 

 the same effect as linen ; only for the positive excitation the stroking 

 had to be very rapid. 



/. "White silk always makes the principal -cleavage -surface of 

 gypsum positive, but the second, which exhibits a vitreous lustre, 

 negative ; v, r hile it makes no difference whether the surfaces of this 

 second cleavage are already present on the piece of gypsum, or are 

 artificially produced by roughly scraping a surface of the principal 

 one. 



Many other substances give opposite electric excitation on friction, 

 according to the circumstances — for instance, mica struck with silk, 

 hardgum rubbed with copper, hardgum whipped with silk, glass 

 struck with silk ; I have not, however, succeeded in discovering posi- 

 tive rules for this. The few experiments cited above (from a to/) 

 even show the impossibility of constructing a series of intensities 

 for frictional electricity. If two bodies are rubbed against one 

 another, the electricity excited in each of them may change into 

 the opposite as the pressure, the velocity, or the direction of the 

 rubbing motion, &c. varies. — Wiedemann's Annalen, 1878, No. 9. 



