LESSONS IN ELECTRICITY. 



experiment, flashing forth at every spark 

 as a spheriod of brilliant golden Jight. 

 The manner in which the lemon is mount- 

 ed on the brass stem B is shown in fig. 

 47. The spark occurs at 5, in the interval 

 between the stems A and B. A row of 

 eggs in a glass cylinder is also brilliantly 

 illuminated at the passage of every spark 

 from a Ley den jar. 



25. Duration of the Electric Spark. 



The duration of the electric spark is 

 very brief ; in a special case Sir Charles 

 Whcatstone found it to be jrj^^tli of a 

 second. This, however, was the maxi- 

 mum duration. In other cases it was 

 less than the millionth of a second. 



When a body is illuminated for a in- 

 stant, the image of the body remains 

 upon the retina of the eye for about one- 

 fifth of a second. If, then, ji body in 

 swift motion be illuminated by an instan- 

 taneous flash, it will be seen to stand 

 motionless for one-fifth of a second at 

 the point where the flash falls upon it. A 

 riiie bullet passing through the air, and 

 illuminated by an electric flash, would 

 be seen thus motionless ; a, circle like D 

 D', fig. 48, 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 motion- 

 less, with all its sectors revealed. A fall- 

 ing jet of water, which appears contin- 



uous, is resolved by 'the electric flash 

 into its constituent drops. Lightning, 

 as shown by Professor Dove, is similarly 

 rapid in its discharge. 



For a long time it was found almost 

 impossible to ignite gunpowder by the 

 electric spark. Its duration is so brief 

 that the powder, when the discharge oc- 

 curred in its midst, was simply scattered 

 violently about. In 1787 Wolff intro- 

 duced 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 

 unrelarded sparks are ignited when the 

 discharge is retarded by a tube of water. 

 A wetted string is the usual means re- 

 sorted to for retardation when gunpow- 

 der is to be discharged. 



The instrument usually employed for 

 the ignition of powder is the universal 

 discharger. We make our own dis- 

 charger thus : i and ^-(fig. 49) are in- 

 sulating rods of glass or sealing-wax, 

 supporting two metal arms, the ends of 

 which can be brought down upon the 

 little central table s. One of the metal 

 arms of the discharger being connected 

 by a wire e with the eartL, the separated 

 ends of the two arms are surrounded 

 ^Yith powder B. Sending through it the 

 unretarded charge, the powder ia scatter- 



FIQ. 49. 



