Experiments extending the circuit

The circuit was extended in two different ways, the first was to add more or bigger Piles to it. The second way to extend the circuit was to increase its length. In the case of Volta the first-mentioned experiments were just the typical experiments with the Pile, whereas the latter were done already before the Pile, with the bi-metallic Pair. Adding the pairs developed the model of contact electricity and the concept of voltage, whereas lengthening the circuit meant developing the model of electric fluid and the concept of an electric current.

5.1.4.1 More Piles in the circuit – stronger voltage

Because of its renewed construction Volta’s Pile enabled systematic studies concentrating on the strength of effects. It was easy to add components to the Pile and the effects produced were much stronger than those produced by the bi-metallic pair.

In Volta’s experiment with a Pile consisting of twenty metallic couples it was observed that a strength of a shock depended on the length of the column in the Pile. If a researcher’s one hand touched the bottom of the Pile and the other hand was moved upwards the Pile (see Figure 21), the following observations were made:

By touching then the fifth, the sixth, and the rest of in succession till I come to the last, which forms the head of the column, it is curious to observe how the shocks gradually increase in force.

Volta 1800, 293

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Figure 21 "The shocks gradually increase in force". In this Pile-experiment a person compares the shocks he feels. While holding his left hand on a bottom of the Pile he moves the right hand gradually higher and receives strengthening electrical shocks.

This observation showed unambiguously that there was a correlation between the number of metallic plates and the strength of the shock received. However, the strength of the electric power was not yet enough for research purposes. To get stronger effects the Pile’s column had to be lengthened. By doubling the length of the Pile the effects really increased in force:

…the shocks…will be much stronger, and extend to both arms as far as the shoulder…

Volta 1800, 294

…this column, formed of forty or fifty couples of metals, which gives shocks more than moderate to both the arms of one person, is capable of giving sensible shocks also to several persons, holding each other by the hands (sufficiently moist) so as to form an uninterrupted chain.

Volta 1800, 294-295 Nicholson’s article reports more exact results of the strength of the electrical power:

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The commotion is stronger the more numerous the pieces. Twenty pieces will give a shock in the arms… One hundred pieces may be felt to the shoulders.

Nicholson 1800, 180 Besides the size of a single Pile, there was another way of producing stronger effects.

In Volta’s picture (see Figure 22) are depicted the idea of adding more Piles in series to increase the electric power of a circuit.

Figure 22 Piles in series (Volta 1800), cf. Figure 21.

With the Chain (the chain of cups, see section 5.1.2, Figure 19) Volta realized the following experimental arrangement, which he used to study the effects resulting from turning around parts of the metal pairs:

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Let three twenties of these tumblers be ranged, and connected with each other by metallic arcs, but in such manner, that, for the first twenty, these arcs shall be turned in the same direction; for example, the arm of silver turned to the left, and the arm of zinc to the right;

and for the second twenty in a contrary direction, that is to say, the zinc to the left, and the silver to the right: in the last place, for the third twenty, the silver to the left, as is the case in regard to the first.

Volta 1800, 297 This kind of arrangement is equivalent to a modern circuit, where some of the batteries in series are connected in reverse polarity. The goal of the experiment was to study how turning around of metal arcs affects the strength of the electric power in a circuit. The experiment was realized so that one hand of a researcher was immersed in the first tumbler of water and with the other one he touched every metallic arc in turn. As a result it was observed that:

...the shocks will gradually increase in force to the twentieth arc, that is to say, to the last of those turned in the same direction; but by proceeding onwards to the 21^st, 22^rd, 23^rd…the shocks will each time became weaker, so that at the 36^th or 37^th, they will be imperceptible, and be entirely null at the 40^th, beyond which…the shocks will be imperceptible to the 44^th or 45^th arc; but they will begin to become some sensible, and to increase gradually, in proportion as you advance to the 60^th, where they will have attained the same force as that of the 20^th arc.

Volta 1800, 297-298

5.1.4.2 Longer circuits – weaker current

Experiments to compare the strength of the electric current had been done already before the Pile. In fact, the taste-experiments (see Figure 18, section 5.1.1) done with the bi-metallic pair nearly a decade earlier included already a viewpoint of extending the circuit.

Longer circuits by taste-experiments

Volta reported the results of the taste-experiments as follows:

Experiments of this kind succeed equally well with two, or three, or a greater number of persons, who make a circular chain; only the effects become more weak in proportion as the circuit through which the electric fluid must pass is longer…

VO, I: 206 So, the idea of the experiments was to extend the electric circuit, and to study its effect on the strength of the electric current. The tongue of one of the subjects was used as a detector of the electric current, and the strength of the current was studied by varying the number of persons in the circuit. In this experiment the electric current was observed via acid sensations in the tongue and also through convulsions in a prepared frog. The

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presence of a frog was necessary because it was the "standard detection device" in most previous investigations on static electricity and thus formed the basis for identification for the similarities of direct current and static electricities.

Children’s experiments with different lengths of platinum wires

The most well known systematic research to construct longer circuits and to investigate the effects of the electric current in the circuit of the Pile was carried out by J. G. Children from the year 1808 onwards. For example, in one series of these experiments, platinum wires of different lengths were circuited with Volta’s Pile as follows:

Experiment 1. Eighteen inches of platina wire, of 1/30th of an inch diameter, were completely fused in about twenty seconds.

Exp. 2. Three feet of the same fire were heated to a bright red, visible by strong day-light.

Exp. 3. Four feet of the same wire were rendered very hot; but not perceptibly red by day-light. In the dark, it would probably have appeared red throughout.

Exp. 5. On iron wire, of about 1/70th of an inch diameter, the effect was strikingly feeble.

It barely fused ten inches, and had not power to ignite three feet.

Children 1808, 33 As it appears from the quotation, there were two variables that Children was interested in. The first one was the length of the wire. The second variable was the material (platinum and iron) of the wire. The results obtained were again clear indications of the dependence between the length of the wire and the observed actions of the electric current.

However, Children did not comment on this dependence, although from the experimental set-up it is evident that he was aware of the dependency. Instead, his interest focused on those aspects of DC-circuit phenomena, which depended on the differences of the materials. He recognized that the main reason behind different results in heating could be assigned to differences in the conductivity of the materials. Platinum is a “perfect”

conductor, so it heats more than iron, which is not so good a conductor. (Children 1808, 34-35)