490 M. PoggendorfF on Galvanic Circuits composed of 
consists of a bored copper cylinder 
with two side screws. In this is in- 
serted from the one side the end of 
the connecting wire^ and from the 
other the piece of wire soldered to the 
plate, or instead of that the plate 
itself, which can be so cut that it may 
be inserted in it. When the screws 
are tightened connexion is made. The 
second is constructed of two copper plates, which can be pressed 
against one another by a screw. In order that this may be 
effected without disturbing the plates, the one is provided with a 
pin which fits into an aperture in the other ; this latter has also 
in the centre, on its inner side, a wedge-shaped furrow for its 
whole length. By means of this second clamp, thin plates, to 
which it is not desirable to solder pieces of wire, may be, as will 
easily be conceived, combined with wires, for the better inser- 
tion of which the above-mentioned furrow is made. The two 
clamps in this respect serve perfectly well aU the purposes of 
mercury, without possessing its numerous inconveniences. 
In general the current of a circuit of the kind described has but 
slight intensity ; nevertheless it is always very perceptible with 
a sensitive multiplier. Frequently it was even so energetic that 
the magnetic needle of the instrument beat with violence against 
the pins which were erected at the points 90° to prevent its en- 
tire reversion ; but often the deflections were but feeble, and in 
these cases especially I always took the precaution to exchange 
reciprocally the four plates employed for each experiment, and 
to take the mean from all the readings. In this way, it is true, 
each experiment became four experiments ; but this exchange 
is quite necessary, as we never find, especially in plates of non- 
noble metals, iron and zinc, two homogeneous to such a de- 
gree, even when cut out of the same mass, as not to produce, 
when immersed alone in a conducting fluid, a frequently con- 
siderable current, which might easily overpower that which 
is intended to be observed. 
Only in some cases where I was already acquainted with the 
direction of the current, for instance on repeating some experi- 
ments which I had previously performed, was I satisfied with 
employing the zinc plates in the combination, so that their 
heterogeneity should act against the current, [instead of exchan- 
ging them. If the current was merely weakened by this, and 
not reversed, the result can be noted as certain. 
I had hoped that the heterogeneity of the zinc would be less 
with the distilled metal, but found it nearly quite as great as 
with the common zinc, and even, as with the latter, increasing 
