CHAPTER XIX. 



VELOCITY OF ELECTRICITY EXPERIMENTS THE ELECTRIC EGG 



FORCE OF THE ELECTRIC SPARK. 



WE are now acquainted with many facts concerning electricity, and have 

 seen that electrical phenomena can be produced by the Electric Machine and 

 the Leyden Jar. (An insulating stool a stool with glass legs is a very 

 desirable adjunct for those who wish to experiment with the machine). 

 Glass is a great insulator, or non-conductor, as a Russian philosopher found 

 to his cost. He had an iron lightning-conductor from his house into his 

 room, the end not connected with the earth but with a glass. One day the 

 lightning came down the rod and reached the glass ; had a communication 

 been made with the earth by a chain, or directly, no mischief would have 

 ensued. As it happened, however, the current was checked by the glass, 

 and immediately darted towards him ; it struck him in the head, and killed 

 the poor man on the spot. If no insulating stool were used, the body charged 

 would be discharged upon contact with the ground. 



The velocity of electricity is very great, and experiments have fre- 

 quently been made. Wheatstone undertook to ascertain the speed of the 

 electric fluid, and the instrument he employed he called a " Chronoscope." 

 He caused a mirror to revolve with enormous velocity, and measured the 

 speed by the vibrations of air, which produced a certain note by the 

 same motive power. (We know already that certain notes are produced by 

 a certain number of vibrations per second.) Wheatstone's Chronoscope 

 consisted of this mirror, in front of which was placed a circular block of wood, 

 in which, in a row, were set six wires carrying small knobs ; round these and 

 over the wood he put an insulating varnish. A Leyden jar was connected 

 outside with the first knob ; between the second and third a quarter of a mile 

 of copper wire was coiled, and a like length of wire between the fourth and 

 fifth ; the inside of the Leyden jar was then connected with the last knob, and 

 the spark passed ; it ran from knob to knob over the long coils of wire. If 

 all the flashes over the wire and knobs occurred simultaneously the mirror 

 would show them side by side ; if not, as the mirror turned a trifle, the dif- 

 ference would be observed. The mirror did show a slight retardation in 

 the passage of the flash, and from certain measurements and calculations 

 Wheatstone estimated the velocity of the spark to be 288,000 miles a second. 



