362 HISTORY OF ASTBONOMY. 



nouf and Guillemin in 1854, to determine the velocity 

 of the electric current. The following is the principle 

 upon which these experiments are founded. Conceive an 

 insulated metallic wire, a hundred miles or more in 

 length, the two extremities of which are brought close 

 together. Near one extremity of the wire, but not in 

 contact with it, is one of the poles of a battery, of which 

 the other communicates with the ground. Near the other 

 extremity of the wire, but not in contact with it, is the 

 wire of a galvanometer, of which the other end com- 

 municates with the earth. If at the same instant we 

 touch one end of the wire to the battery and the other 

 to the galvanometer, the current runs through the 

 wire, reaches the galvanometer, and deflects the needle. 

 But the current requires a certain time to traverse the 

 wire. If the contacts continue sufficiently long, the cur- 

 rent will reach the galvanometer, and deflect the needle. 

 If the contacts do not continue sufficiently long, the 

 current will not reach the galvanometer, and the needle 

 will not be deflected. By gradually diminishing the 

 time of contact, we may determine the exact interval at 

 which the deviation ceases. This interval is the time 

 which the current requires to traverse the wire. 



The contacts are made by the rapid revolution of a 

 wheel whose circumference consists of narrow strips of 

 wood and brass alternately. 



MM. Guillemin and Burnouf concluded from their 

 experiments that the velocity of the electric current in 

 an iron wire one sixth of an inch in diameter, was 



