56 REPORT—1850. 
New Researches on the Conductibility of the Earth. By Prof. Marreuccti. 
Although the good conducting power of the earth is at present generally admitted, 
and is advantageously applied to the construction of electrical telegraphs, it must be 
confessed that nothing has been hitherto known of the laws and theory of this sin- 
gular phenomenon. In England, Germany and Russia, it has been found advisable, 
for several years past, to form the telegraphic circuit partly with the earth and partly 
with a metallic wire, instead of forming the whole circuit with metal wire only. I 
was, I believe, the first to show by exact experiments made in 1844 at Pisa, and by 
others performed according to my propositions at the Scientific Congress of Milan, 
that the resistance of the earth to the passage of the electrical current, which is 
sensible in short distances, ceases to increase, and remains constant when the distance 
between the electrodes plunged in the earth has attained a certain length. Having 
latterly renewed my studies on this subject, I have confirmed and extended, in a com- 
plete and general manner, the conclusions drawn from my former researches; 1 have 
also demonstrated the principal result, given above, by different experimental pro- 
cesses. I have compared the resistance of a mixed telegraphic circuit with that of 
an entirely metallic circuit, containing a length of wire twice as great as that em- 
ployed in a mixed circuit. I have also formed metallic circuits of very fine brass 
wires, having the same resistance as the metallic portion of a very long mixed tele- 
graphic circuit; and finally, by making use of long metal wires covered with gutta 
percha, I have been able to compare the resistance of an entirely metallic circuit with 
that of a mixed circuit, in which the metallic portion remained constantly the same, 
and to which were added different lengths of earth. The following are the principal 
conclusions drawn from experiments which have occupeid me for about a year :— 
The resistance of a layer of earth to the. passage of the electrical current varies 
according to the quantity of water contained in the earth of which it is composed ; 
according to the specific gravity of that earth; according to its depth beneath the 
surface ; and according to the nature of the electrodes and extent of their surface. 
This resistance does not increase with the increased length of the layer of earth; 
on the contrary, beyond a certain limit of length, which varies according to the dif 
ferent circumstances just indicated, but which in all cases is of little extent, the re- 
sistance of a layer of earth remains constant, whatever be its length. It is unnecessary 
to say, that I could not prove this fact by experiment on circuits exceeding eighty 
“miles in length; such being the average of the telegraphic circuits in Tuscany. 
In making the experiment near the surface of the soil, it is difficult to plunge the 
electrodes in earth of exactly the same conducting power; different portions of the 
surface of soil possessing either better or worse conductibility than that on which we 
begin to operate, it follows that in increasing the distance between the electrodes, 
we may find either an increase or diminution in the resistance of the earth. Like- 
wise, in operating on a long mixed telegraphic circuit, which is not perfectly isolated, 
owing to the effect of the different derived circuits formed between the posts and the 
earth, the electric current is stronger near the pile than at a distance, and stronger 
than in a cixcuit which is formed only of metal wire equal in length to that which 
enters into the mixed circuit. This explains the results which I had obtained from 
my former uncompleted experiments. ‘The resistance of a layer of earth appears to 
diminish, as its length increases, only in case we meet with other layers of better 
conducting power. 
In every layer of earth of a certain constant conducting power, the resistance, 
which at first increases very feebly with the increased length of the layer, becomes 
very soon constant, and continues the same for all the subsequent lengths, however 
great, on which experiments have been made. Now it is evident, that, as the increase 
of resistance in a long metallic circuit is scarcely perceptible when we add to the 
circuit, by means of two large electrodes, a thin stratum of water, so we ought to 
find in the long mixed telegraphic circuits, that the resistance of the earth is null, or 
nearly so, since it is equal to that of a thin stratum of water of a very large section. 
The law of the conducting power of the earth being established, it remains to give 
the theory of this phenomenon. The opinion of the scientific world is divided on 
this point. Some explain the good conducting power of the earth by the almost infi- 
nite section of the earth compared with the distance of the electrodes ; others again 
suppose that the electricities at the two extremities of the pile are dissipated in the 
earth in the same manner as the electricity of the conductor of an electrical machine, 
