n be made in advance of any length whatever, and,
according to local exigencies, it may be easily curved and given the
form of a flat or cylindrical ring of varying width. Even though the
ribbon has already been cut for a ring of given diameter, it may be
still further enlarged by drawing it out and leaving a bit of the ring
open, so as to thus obtain a nearly corresponding diminution in the
resistance. Such a resistance may be still further diminished by
rendering the ring higher, that is to say, by employing an annular
cylindrical form.

After assuring himself, by experiments on a small scale, that
calculation and observation gave concordant results for the flat ring,
the author made an experiment on a larger scale with the annular
network. For practical reasons he employed for this purpose a copper
wire 2.5 mm. in diameter, which may be expected to last as long as one
of iron plate 2 mm. in thickness. Calculation showed that in a ribbon
160 mm. wide, meshes 40 mm. in breadth were advantageous and favorable
as regards rigidity. A reticulated ribbon like this, 4 meters in
length, was made and formed into a flat ring having an external
diameter of 1.42 m. and an internal one of 1.10 m. The resistance of
this ring was found to be W = 0.3485 (1/_k_), and that of a plate one
meter square, W0 = 0.368 (1/_k_).

As the conductivity of the earth is very variable, and as we cannot
have an absolute guarantee that the ramming will be uniform, it seemed
proper to make the measurements of the resistance by fixing the plate
and the ring in succession to the lower surface of a small raft, in
such a way that the contact with the water should correspond as well
as possible to the suppositions made for the calculation. As a second
ground conductor, a system of water pipes was used, and, after this, a
lightning rod conductor, etc.

Repeated and varied experiments gave, for the calculation of the
values of the resistances, equations so concordant that the following
results may be considered very approximate.

The square plate had a resistance of 35.5 Siemens units, and the
reticulated ring one of 32.5. From the first figure we deduce k =
1/91.12, that is to say, the specific conductivity of river-water is
1:91120000. Calculation, then, gives as the resistance of the earth in
Siemens units:

Calculated. Observed.
Square plate. 33.5 33.5
Annular ring.