342 THE POPULAR SCIENCE MONTHLY. 



Sec. 23. Duration of the Electric Spark. The duration of the 

 electric spark is very brief: in a special case, Sir Charles Wheat- 

 stone found it to be 2T^To tn of a second. This, however, was the 

 maximum duration. In other cases it was less than the millionth of 

 a second. 



Fig. 32. Fig. 33. 



When a body is illuminated for an instant, the image of the body 

 remains upon the retina of the eye for a fraction of a second. If, 

 then, a body in swift motion be illuminated by an instantaneous flash, 

 it will be seen to stand motionless for the fraction of a second at the 

 point where the flash falls upon it. A rifle-bullet passing through the 

 air, and illuminated by an electric flash, would be seen thus motion- 

 less ; a circle like D D', Fig. 33, divided into black and white sectors, 

 and rotating so quickly as to cause the sectors to blend to a uniform 

 gray, appears, when illuminated by the spark of a Ley den-jar, per- 

 fectly motionless, with all its sectors revealed. A falling jet of water, 

 which appears continuous, is resolved by the electric flash into its 

 constituent drops. 



For a long time it was found almost impossible to ignite gunpow- 

 der by the electric spark, its duration was so brief; the powder, when 

 the discharge occurred in its midst, was simply scattered violently 

 about. In 1787 Wolff introduced into the circuit through which the 

 discharge passed a glass tube wetted on the inside. He thereby ren- 

 dered the ignition certain. This was owing to the retardation of the 

 spark by the imperfect conductor. Gun-cotton, phosphorus, and 

 amadou, which are torn asunder by the unretarded spark, are ignited 

 when the discharge is retarded by a tube of water. A wetted string 

 is the usual means resorted to for retardation when gunpowder is to 

 be discharged. 



The instrument usually employed for the ignition of powder is 

 called a universal discharger. It is represented in Fig. 34. Zand T 

 are insulating rods of glass or sealing-w T ax, supporting two metal 

 arms, the ends of which can be brought down upon the little central 



