C. GENERAL PHYSICS. 159 



come the resistance offered by the rarefied medium through 

 which it must flow. 



EARTH-CURRENTS ON TELEGRAPHIC LINES. 



The Asiatic Society of Bengal, in consideration of the im- 

 portant labors of Mr. Schwendler, has taken steps to influ- 

 ence the government of India to especially investigate the 

 subject of earth-currents on telegraph lines a work which 

 ought, in the interest both of science and art, to be taken 

 up not only by the European governmental, but by American 

 private telegraph companies. 



ON UNILATERAL CONDUCTIVITY OF ELECTRICITY. 



While en<xao;ed in other work, Dr. Schuster states that he 

 met with an irregularity which seemed to be of such a pecul- 

 iar nature that he subjected it to a separate investigation; 

 although he is not yet able to raise this phenomenon above 

 the rank of an irregularity, yet his experiments leave no 

 doubt as to the fact. It seems to him clear that the current 

 produced by an electro-motive force in a circuit composed 

 entirely of copper wires, joined together by means of bind- 

 ing screens, may under certain circumstances be different 

 from the current produced by the same electro-motive force 

 acting in an opposite direction. He calls this phenomenon 

 "Unilateral Conductivity." The most plausible explanation 

 seems to him to be that a thin layer of air may sometimes 

 intervene between the two wires that are screwed together, 

 an explanation that has been confirmed by some experiments, 

 while others show that it is insufficient. 7 A, XL VIII., 246. 



THE ELECTRIC CHARGE OF A CONDUCTING WIRE. 



The researches of various physicists have proved that an 

 electric current, before it can circulate in any conductor, 

 must charge it electrically, and consequently in the entrance 

 of a current into the circuit two periods are distinguished. 

 In the first the wire is charged, the current passing through 

 a variable state until it gradually acquires its normal value. 

 In the second period the current has become constant, and 

 its value depends on the conditions determined by Ohm's 

 law. According to Villari, the first or variable state has no 

 constant duration ; it increases with the length and condition 



