ELECTRICITY. 389 



that the duration of the light of the spark does not 

 exceed the one-millionth part of a secoud.* 



"When the coDtinuifcy of a substance conducting electricity is interrupted, a 

 spark will be produced at every point where the course of the conductor ia 

 broken. 



A great yariety cf beautiful experiments may bo performed to illustrate 

 this principle. Thus, upon a piece of glass may be placed at a short distance 



from each other any number of bits or 

 Fig. 329. pieces of tin-foil, as is represented by 



jrr" ...rnfllHlllli'iliatiiillWIIIiliPilllHlil ^ig- =^29 ; when the metal at either end 



13 connected with the prime conductor 

 of an electrical machine, the sparks will 

 pass from one piece of tin-foil to the 

 other, and form a stream of beautiful 

 light. By varying the position of the 

 pieces of tin-foil, letters, or any other devices may be exhibited at the pleasure 

 of the operator. 



In a like manner, by fasten- 

 FlG. 330 ing by means of lac- varnish a 



spiral line of pieces of tin-foil 



[jT ^<,'j^ ^j^ ^<^ ,o'' ^,^''' ,--'' ,,o'" ^' ^.'""rfi upon the interior of a tube, as 

 yy ^" '"" ^'' • ^'' - — • ^' • ''^' — ^ — ^ — mJ is represented in Fig. 330, a 



serpentine line of fire may be 

 made to pass from one end of the tube to tho other. 



• The arrangement by which this fact was demonstrated by Mr. "Wheatstone of England, 

 may be described as foUowB: — Considerable lengths of copper wire (about half a mile 

 being employed), are so arranged, that three small breaks occur in its continuity — one near 

 the outer coating of a Leyden jar, one near the connection with the inner coating, and 

 another exactly in the middle of the wiro — so that three sparks are seen at every dis- 

 charge, one at the break near the source of excitation, another in the middle of its path, 

 and the third close to the point of returning connection ; these, by bending the wire, are 

 brought close together. Exactly opposite to this was placed a metallic speculum, fixed 

 on an axis, and made to revolve parallel to the line of the three sparks. When a spark 

 of light is viewed in a rapidly revolving mirror, a long line is seen instead of a point. It 

 will be obvious that three lines of light will be seen in the revolving mirror every time a 

 discharge takes place, and that if the first or the last differ in the smallest portion of time, 

 these lines must begin at different points on the speculum. 



When the mirror revolved slowly, the position of the lines was uniform, thus ^^ ' 



bnt when the velocity was increased, they appeared thus ' ; those pro- 



duced by the sparks at either end of the wire being constantly coincident, but the spark 

 evolved at the break in the middle being slightly behind the other two. From this, it 

 appears that the disturbance commences simultaneously at either end of a circuit, and 

 travels toward the middle. This has been adduced in proof of the two electricities. It 

 was thus determined that electricity moves through copper wire at a rate beyond 26S,000 

 miles in a second. It will be evident to any one considering the subject, that the length 

 of the line seen in the speculum depends on the duration of the spark. Wlien the mirror 

 was made to revolve SOO times in a second, the image of the spark, at 10 feet distance, 

 appeared to the eye of the observer to make an arc of about half a degree, and from this 

 its duration was calculated. — ifu»i^ 



