ELECTRICITY AND MAGNETISM 



131 



reaches the eye instantly. The noise it makes travels 

 more slowly. The velocity of sound in air is about 1,100 

 feet per second. If one second elapses between the flash 



FIG. 65. JOSEPH HENRY: PHYSICIST. 1797-1878 



He was a born experimentalist: one who knew how to cross-examine 

 Nature as an astute lawyer would cross-examine a witness and thus bring 

 out her inmost secrets. He was one of those men by whom it seems as if 

 Nature loves to be cross-examined. SIMON NEWCOMB in Leading American 

 Men of Science. 



and the report, the discharge is 1,100 feet away, or more than 

 one fifth of a mile. (One can count five in one second.) 



142. Current or Voltaic Electricity. About the begin- 

 ning of the nineteenth century, it was learned that if two 

 different metals, such as copper and zinc, are placed in a 

 weak acid and connected by a wire fastened securely to the 

 metals, a current of electricity will pass through the wire. 

 Carbon, and a metal upon which the solution acts chemically, 



