272 ON EARTH CURRENTS, AND THEIR CONNEXION 



that the diurnal variation of the horizontal magnetic needle is due 

 to electric currents traversing the earth's crust. 



2. The first and most important step towards the result above 

 referred to has been made by Mr. Barlow. In a Paper* com- 

 municated by him to the Royal Society in the year 1848, he 

 established the important fact, that a wire, whose extremities are 

 connected with the earth at two distant points, is unceasingly 

 traversed by electric currents, the intensity of which varies with 

 the azimuth of the line joining the points of contact with the 

 ground. The direction of these currents was proved to be the 

 same at both extremities of the same wire, and was shown to 

 depend on the relative positions of the Earth-connexions, while 

 it was wholly independent of the course followed by the wire 

 itself. The currents cease altogether when either of the contacts 

 with the earth is interrupted. From these facts Mr. Barlow 

 concluded that " the currents are terrestrial, of which a portion is 

 conveyed along the wire, and rendered visible by the multiplying 

 action of the coil of the galvanometer." 



Mr. Barlow further observed that, apart from sudden and 

 occasional changes, the general direction of the needle of the 

 galvanometer appeared to exhibit some regularity. He was thus 

 led to institute a series of observations for fourteen days and nights 

 simultaneously on two telegraphic wires, one connecting Derby 

 and Eugby, and the other connecting Derby and Birmingham, 

 the positions of the needles in both circuits being recorded every 

 five minutes, day and night. From these observations he con- 

 cluded 



" 1. That the path described by the needle consisted of a 

 regular diurnal motion, subject to disturbances of greater or less 

 magnitude. 



" 2. That this motion is due to electric currents passing from 

 the northern to the southern extremities of the telegraph wires, 

 and returning in the opposite direction. 



" 3. That, exclusive of the irregular disturbances, the currents 

 flowed in a southerly direction from about 8 or 9 A.M. until the 

 evening, and in a northerly direction during the remainder of the 

 twenty- four hours." 



* " On the spontaneons Electrical Currents observed in the wires of the Electric 

 Telegraph." Phil. Trans., 1848. 



