RECENT PROGRESS IN PHYSICS. 359 



I § T2. Electrical figures. — By means of electricity, figures can be 

 produced on the surface of different bodies, which are either directly 

 ivisible or are rendered visible by strewing dust^ or by breathing upon 

 ;ieni. Riess has made an extensive series of experiments (Pog. Ann., 

 LXIX, l"! on these phenomena, the best known of which are the 

 Lichtenberg figures, and he has determined very accurately the cir- 

 cumstances under which these figures and images appear. 

 j Riess divides them into primary electrical delineations, or such as 

 are caused by difi'erent parts on the surface of poorly conducting sub- 

 stances being placed in unlike electrical condition, and becoming 

 visible on being sprinkled with powders ; and 



[ Secondary electrical delineations, which are produced when the film 

 pf foreign matter which covers nearly all bodies is affected by the 

 electrical discharge ; in this case the figures are made to appear by 

 breathing upon the plate, or else visible marks may appear imme- 

 diately, if the surface of the body itself has been in any way attacked. 

 I We shall first consider the figures made visible by sprinkling pow- 

 |ier upon them. 



1 § 73. Dust figures. — To produce the Lichtenberg figures Riess used 

 square copper plates, covered on one or both sides with a coat of pitch 

 ubout i line thick. 



j The formation of dust figures (Lichtenberg figures) is a consequence 

 pf the electroscopic action of electrified spots on the resinous surface 

 upon the powder itself, electrified by shaking in the bag through 

 which it is sifted. A mixture of flour of sulphur and minium is best 

 jfor this purpose. Positively electrified places on the plate are covered 

 iwith the sulphur, and therefore appear yellow ; the minium, on the 

 contrary, is collected on the negative spots, which thus ap])ear red. 

 I The spark having passed over the pitch surface, so that a dust 

 l^gure would have appeared if it had been immediately dusted, no 

 figure will be formed if the pitch surface is first exposed for a second 

 ^o the flame of a spirit lamp, by which the electricity is removed from 

 ^he plate. 



. The simplest mode of producing dust figures is the following, used 

 also by Riess : A copper plate, covered on one side only with pitch, is 

 •touched by a conductor, and an insulated metallic point is placed on 

 "the pitch surface. The upper end of the point being touched by the 

 knob of B. -positively charged jar, remove the insulated point, and on 

 [powdering with the above described mixture a round yellow sun, with 

 idense rays, will appear. 



i The experiment being conducted in the same manner with a nega- 

 tively charged jar, a perfectly red circular disk will appear. 

 : This diversity in the appearance of the figures is well known ; but 

 ^Riess has directed attention to another remarkable distinction, namely, 

 [that the positive figure is much larger than the negative, though 

 iequally strong charges have been used. 



[ With a given 'positive charge of the jar the yellow sun had (as a 

 imean of 3 experiments) a diameter of 16.1 millimetres. 



With an equally strong negative charge the red disk had a diame- 

 ter (also a mean of 3 experiments) of 5.8 millimetres. 



The diameters of the negative and positive figures, produced by 



