324 VELOCITY OF ELECTRICITY. [SECT, xxvnt. 



enough advanced to enable him to state its absolute celerity, 

 he has ascertained that it much surpasses the velocity of light. 



In the horizontal diameter of a small disc, fixed on the wall 

 of a darkened room, are disposed six small brass balls, well 

 insulated from each other. An insulated copper wire, half 

 a mile long, is disjointed in its middle, and also near its two 

 extremities ; the six ends thus obtained are connected with 

 the six balls on the disc. When an electric discharge is sent 

 through the wire by connecting its two extremities, one with 

 the positive, and the other with the negative coating of a 

 Ley den jar, three sparks are seen on the disc, apparently at 

 the same instant. At the distance of about ten feet, a small 

 revolving mirror is placed so as to reflect these three sparks 

 during its revolution. From the extreme velocity of the elec- 

 tricity, it is clear that, if the three sparks be simultaneous, 

 they will be reflected, and will vanish before the mirror has 

 sensibly changed its position, however rapid its rotation may 

 be, and they will be seen in a straight line. But if the three 

 sparks be not simultaneously transmitted to the disc if one, 

 for example, be later than the other two the mirror will 

 have time to revolve through an indefinitely small arc in the 

 interval between the reflection of the two sparks and that 

 of the single one. However, the only indication of this small 

 motion of the mirror will be, that the single spark will not 

 be reflected in the same straight line with the other two, 

 but a little above or below it, for the reflection of all three 

 will still be apparently simultaneous, the time intervening 

 being much too short to be appreciated. 



Since the number of revolutions which the revolving 

 mirror makes in a second are known, and the angular 

 deviation of the reflection of the single spark from the re- 

 flection of the other two can be measured, the time elapsed 

 between their consecutive reflections can be ascertained. 

 And, as the length of that part of the wire through which 

 the electricity has passed is given, its velocity may be found. 



