ELECTRIC EXPERIMENTS. 



length of the spark be not great, the spark will be straight ; if it 

 be made somewhat greater, it takes a sinuous and forked form, as 

 though floating dust particles served as stepping-stones and rendered 

 a crooked path the easiest. If the charge be very powerful, the 

 spark will take the zigzag form so familiar in the lightning-stroke. 

 In the dark, the continuous discharge into the air produces a 

 luminous appearance at the ends of the conductor. This appear- 

 ance, known as a brush, may be im- 

 proved by holding a large, smooth, 

 metal globe at a distance a little too 

 great for the passage of a spark. If 

 the conductor be provided .with a point, 

 the point will glow when the machine 

 is worked in the dark. 



(25.) Divide a circle into black and 

 white sectors, as shown in Fig. 173, and 

 attach it to a whirling table ( 74). 

 Revolve it so rapidly that the colors 

 FIG. 173. blend and the disc appears a uniform 



gray. Darken the room and illuminate 



the rapidly revolving disc by the electric spark from a Ley den jar. 

 The disc will appear at rest, and each sector will appear separate 

 from its neighbors. This shows that the duration of the electric 

 spark is less than the persistence of vision. 



(26.) In a dark room, place a piece of loaf sugar in contact with 

 the outside coat of a charged Leyden jar. Place 

 one knob of the discharger upon the sugar, and 

 bring the other near the knob of the jar. When 

 the jar is discharged thus through the sugar, the 

 sugar will glow for some time. 



(27.) The " luminous jar," represented in Fig. 174, 

 is a modified Leyden jar. The outer coat consists 

 chiefly of a layer of varnish sprinkled over with 

 metallic powder. A strip of tinfoil at the bottom 

 affords means of communication with the earth. A 

 similar band at the upper edge of the outer coat is 

 provided with an arm, as shown in the figure. The 

 rod of the jar is curved so as to bring the knob near 

 the projecting arm of the outer coat. The jar 13 

 suspended by the curved rod from the prime conduc- FIG. 174. 

 tor, and its lower strip of tinfoil connected with the 

 earth. When the machine is worked, sparks pass between the 

 knob and the projecting arm. In a dark room, the metallic powder 



