Experiments of electric circuit by the bi-metallic pair

The predecessor of the Pile, the bi-metallic pair, was devised during the modelling debate between Italian researchers Luigi Galvani and Alessandro Volta. The first version of the pair was introduced by Galvani in 1791, and already the next year Volta rose to the challenge, first by repeating Galvani’s experiments and then modifying them (Kipnis 2003, 18-19; Kragh, 2000, 134). During the combat, from 1791 to the death of Galvani in 1798, many experiments were carried out as both researchers tried to vindicate their own models of the nature of electricity and its origin. Galvani’s model was called the animal electricity model. Volta did not agree with Galvani’s arguments and he constructed a contact electricity model to describe his conception. Besides modelling the nature of electricity and its origin, these experiments also widely modelled the closed circuit model (see 5.2).

Galvani’s experiments

Galvani’s model was based on the following types of experiments: 1) Leyden jar experiments, 2) railing experiments, and 3) bi-metallic pair experiments. The first version (1780-1781) of his experiments was to apply electric shock to a prepared frog for instance by the Leyden jar (Bresadola 1998, 373-374). When the circuit was closed the muscles of the frog were seen to convulse and contract. The next experiments, which were reported in 1791, did not include an external (known) electrical source anymore. The circuit consisted of the prepared frog, metal pieces and arcs. The so-called railing experiment was described as follows:

…The frogs prepared in the usual manner horizontally over the railing. Their spinal cords were pierced by iron hooks, from which they were suspended. The hooks touched the iron bar. And, lo and behold, the frogs began to display spontaneous, irregular, and frequent movements. If the hook was pressed against the iron surface with a finger, the frog, if at rest, became excited – as often as the hook was pressed in the manner described.

Pera 1992, 81

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Figure 16 The prepared frog over the railing (Pera 1992, 82). Frog's spinal cords were pierced by iron hooks, from which they are suspended. When the hook of the spinal cord touches the iron bar the frog began to convulse.

Galvani also did bi-metallic pair experiments and in fact repeated the railing experiment with a bi-metallic arc. The new way to do the same experiment was simply to connect the sciatic nerve and the leg of a prepared frog to a bi-metallic arc in order to get the leg to twitch (Kipnis 2003, 18).

Volta’s first experiments against Galvani’s model

Volta noticed Galvani’s experiments and especially the way they were explained.

Galvani’s final conclusions that the animal acts as a source of discharge (Pera 1992, 82) were a starting point to Galvani’s and Volta’s big controversy on the nature and source of electricity. Galvani’s opinion and explanation of the experiments was that animals contain a specific electrical fluid called animal electricity (Pera 1992, 77, 85; Gill 1976, 353).

Volta reproduced Galvani’s experiments, but after them and his own research, he could not agree with Galvani’s model (Gill 1976, 352). Instead, this contradiction was a starting point to his model of contact electricity, which regarded the contact of two different metals as the cause of observed contractions.

In his struggle against Galvani’s model, Volta tried different modifications of his circuit. He used circuits, in which two objects of dissimilar metals connected with salt-water liquid or with the prepared frog formed a so called bi-metallic. The goal was to show that the prepared frog was not an essential part of the circuit, but only a sensitive detector of electricity. One attempt was to eliminate the prepared frog from the circuit and replace it with so-called Nicholson’s doubler. However, the doubler was known to create its own electricity, so the experiment was not so persuasive (Kipnis 2003, 19-20).

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Volta’s experiments with the bi-metallic pair to compare the strength of electromotor of different conductors

The aim of another series of experiments was to compare the strength of the electromotor of different conductors: bi-metallic pairs, single metals and two second class conductors.

According to Volta’s modified model, conductors could be classified into two groups: 1) conductors of the first class (metals and some other solids), 2) the second class (liquids or a humid bodies). In these experiments the prepared frog was used as a detector of the electricity: the stronger the contractions the stronger electromotor of the contact. The results were encouraging to Volta: the effects were strongest in the circuit of bi-metals, clearly weaker in the circuit of a single metal and two second class conductors, and only just detectable by very sensitive frogs in circuits of pure second class conductors (Kipnis 2001, 123-124).

Figure 17 The frog as a part of Volta's circuit (VO, I: 104). The circuit consists of two different metal disks, which are combined with a metal arc and a prepared frog. In combining the metal disks Galvani used a prepared frog, whereas Volta thought that the connecting part could be any wet body.

Taste experiments by the bi-metallic pair

Volta’s so-called taste experiments (1793) are also an important example of bi-metallic pair experiments. In these experiments the equipment was composed of two glasses of water, of which the other included a plate of silver and the other a plate of tin (the bi-metallic Pair). The plates were connected by another metal. The circuit, shown in Figure 18, was a closed chain formed by persons touching each others, and one of them at the end of the chain dipped his tongue into the basin of water (tin), whereas the person at the other end immersed his finger in the other basin of water (silver). (VO, I: 206)

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Figure 18 Volta's taste experiment. The circuit consists of tin and silver plates (a. and b.), a connecting metal arc (f.), human’s tongue (c.), a wet and prepared frog (d.), basins (e.) and a chain of people.

This kind of connections finally led to the rejection of the body of a frog as a necessary part of a circuit. This also produced proof against Galvani’s animal electricity model. As the new invention, the bi-metallic pair, showed that the nerves of an animal were not a source or a cause of electricity, it was time to explain the convulsions in a new way.