490 M. PoggendorfF on Galvanic Circuits composed of 



Fig. 2. 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 distui^bing 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 all 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 M^as 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 



