The methods of physics, back to the map
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EVIDENCE FOR USE OF M6: MILL'S METHODS
Faraday (1914) uses the methods repeatedly in his electro-chemical researches. He hypothesised that the electric power of a voltaic battery (E) was caused by chemical action. The competing theory - research programme - was the contact theory. He used the accepted consensus G connecting chemical action and heat, such that, for example, heating a metal in an acid increases the rate of reaction ie. changing H changes C. So he heated one terminal of a liquid cell, to see if this variation in H would produce a concomitant variation in E (ie. via C). Indeed it did. (p.281 para. 944) "I cannot but view in these results of the action of heat, the strongest proofs of the dependence of the electric current in voltaic circuits on the chemical action of the substances constituting these circuits". Then he tries dilution; his positive heuristic, as he links his theory in with propositions in the consensus, indicates that diluting the liquid in the voltaic cell should reduce E, since its effect on a chemical reaction is to reduce it (G: consensus). He predicts that changing dilution will (p.284 para.957) "produce some corresponding change in the voltaic pile". Then he shows experimentally that it does. This is not a dramatic discovery, since Avogadro and Oersted have already indicated that they had observed such an effect (para. 957), but it is still support for his theory; he is using Mill's methods to suggest that a concomitant variation is strong evidence of the true existence of a causal connection.
Faraday was not a hair-splitting philosopher; we should interpret 'strongest proofs' sympathetically.
The methods of physics