Page:Popular Science Monthly Volume 9.djvu/364

342 23. Duration of the Electric Spark.—The duration of the electric spark is very brief: in a special case, Sir Charles Wheatstone found it to be $1⁄24000$th of a second. This, however, was the maximum duration. In other cases it was less than the millionth of a second.

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 motionless; 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 Leyden-jar, perfectly 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 gunpowder 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 rendered 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. I and I'  are insulating rods of glass or sealing-wax, supporting two metal arms, the ends of which can be brought down upon the little central