ON THE ELECTRICAL CURRENTS OF THE EARTH. 309 



case of a mixed line, when, the distance between the extremities remaining- tlie 

 same, the tenninal cavities which constitute the conmiunication between the wires 

 and the earth are at different levels, than when these communications are estab- 

 lished in a horizontal plane. For the verification of this, 1 have established on 

 the heights of Turin a line whose wire, in a straight direction, has a length of 

 scarcely 600 metres, while the terminal cavities have a difference of level of 

 nearly 150 metres. The line which joins the two cavities is in an intcnnediate 

 direction, or southeast and northwest. The current has circulated constantly, 

 for five or six months, from below upwards in the wire, or from the northwest to 

 the southwest extremity. All the precautions which I have before described 

 were observed in the construction of the cavities in which the plates of zinc are 

 sunk, and I am certain that the current obtained depends neither on any hetero- 

 geneousness in the wire, nor on the terminal plates, nor on a chemical action 

 between the plates and the terrestrial strata in which they are imbedded. When 

 care is taken, as I have practiced for several days in succession, to maintain at a 

 constant height tlie liquids of the terminal cavities, that is to say, the water and 

 the solution of sulphate in the porous vessels, the deviation remains nearly invari- 

 able, whatever may l)e the state of the sky and temperature of the air, and only 

 after quite a long rain has the deviation temporarily increased. In this line 1 

 have not remarked the periods of which I have spoken. Other lines of nearly 

 the same length, established in similar fomiations at the foot of the hill on a 

 horizontal plane, yielded no sensible deviation. 



If the influence of the difference of level of the extremities of the metallic line 

 should be verified in a great number of different cases, if the direction of the 

 cmTcnt in the wire should prove constant, that is to say, always from below 

 upwards, might we not be tempted to attribute these currents to the negative 

 electric state of the earth, the tension of which is then unequal between the plain 

 and the elevated points, as w^e find in an electrified globe communicating with a 

 metallic point? As the signs of the positive electricity of the air are seen in 

 effect to augment in proportion as we ascend in the atmosphere, so also are the 

 signs of negative electricity found to be stronger in ascending, when an isolated 

 copper wire, one extremity of which communicates with the earth, is carried Avith 

 the other extremity in contact with the ball of the electroscope. This explana- 

 tion might be submitted to proof when the atmosphere presents for a certain 

 lime signs of negative electricity. I have sometimes obtained very transient 

 signs of this electricity at the approach of storms of rain, without noticing any 

 variation in the current of the line. 



My chief object has been to investigate the relation which exists between 

 these currents and atmospheric electricity, and next, to verify the result obtained 

 and described in the first part of this memoir, by studying these currents in 

 lines, the extremities of which are sunk in the earth at different levels. The 

 first experiments were made upon the line above described, between the hill of 

 Turin and the adjacent plain. The extremities, as has been already said, were ter- 

 minated by plates of amalgamated zinc immersed in a saturated solution of sulphate 

 of zinc contained in a porous vessel, which was plunged in turn in the water of a 

 sort of capsule excavated from one to two metres below the surface of the earth. 

 This mode of constructing the mixed* line is the only one which yields sure and 

 constant results, and I would advise all physicists who occupy themselves with 

 the subject not to deviate from it. The water which filled the two cavities was 

 the same, and care was taken to maintain it at a constant level. Dming several 

 days of July, in the present year, I have continued to observe from hour to hour 

 the deviations of the galvanometer inserted in this line; the current was always 

 an ascending one in the wire, though I changed several times the position and 



* By this term we uuderstand a circuit composed of an extended wire and the strata of earth 

 intervening between its extremities. — J. H. 



