Benjamin Franklin Papers

To Benjamin Franklin from Gaspard-Bonaventure-Timothée Ferry: Résumé, 28 March 1783

From Gaspard-Bonaventure-Timothée Ferry:2 Résumé

ALS: American Philosophical Society

<Marseille, March 28, 1783, in French: You will probably be surprised that a young man barely 20 years old, a runt in the field of physics, would take the liberty of writing to you. I am motivated by the spirit of emulation, which obliges great men to enlighten those who ask for instruction. My teacher, père Aubert, favors your view of the Leyden jar over that of the abbé Nollet. While I was initially skeptical, I eventually came to the same opinion. Still, while I regard your theory of attraction and repulsion as one of the most beautiful discoveries, I have the audacity to claim that it is not supported by physical explanations.3 I base this assertion on what appears starting on page 208 of d’Alibard’s 1756 translation of your letters. [Ferry summarizes and quotes at length passages about the nature of glass and how it receives and repels electrical fluid.]4 I confess that I never liked abstract ideas in physics, including the terms attraction and repulsion. M. Nollet demanded that you explain your theory on the basis of some solid reason derived from the nature of the bodies. I now offer such an explanation: glass itself is elastic, even in the middle layer where the pores are smallest. The Leyden shock results from the elastic spring of the inner surface of the glass, which is bursting with electrical fluid. Moreover, the elasticity of glass can also explain the central mystery of the Leyden jar: why one cannot charge it when the jar is insulated. If the jar is insulated, the outer surface must retain its electrical fluid. Therefore, it does not allow the inner surface to expand and take in a quantity of excess electrical fluid. The elasticity also explains why the jar works better when filled with hot water than with cold (hot water dilates the pores of the glass) and why thin glass works better than thick glass (thin glass is more elastic).

If you deign to honor me with a response, please give the reply to the person who delivered the present letter.5 And, because someone who knows I wrote to you might try to send me a false reply, please put on the address sheet this distinctive mark [an infinity symbol].>

[Note numbering follows the Franklin Papers source.]

2This 18-page critique of BF’s theory, written when Ferry was enrolled in a collège run by Oratorians, is full of flattery and false modesty. At the end of 1783 he wrote a shorter set of observations on the abbé Nollet’s electrometer, which was published in the Jour. de physique, XXIV (1784), 315–18. Ferry became a professor of experimental physics and also published works of poetry: Quérard, France littéraire.

3Though Ferry proclaims his own audacity, his response was typical of many French scientists’ reaction to BF’s theory of electrical action, namely that BF did not offer “physical causes” for attraction and repulsion, or, indeed, for any electrical effect. For a detailed analysis of the differences between BF’s and Nollet’s electrical theories see Jessica Riskin, Science in the Age of Sensibility: the Sentimental Empiricists of the French Enlightenment (Chicago, 2002), pp. 68–103, and J. L. Heilbron, Electricity in the 17th and 18th Centuries (Berkeley, Los Angeles, and London, 1979). We are indebted to Prof. Riskin for her help in summarizing Ferry’s argument.

4Benjamin Franklin, Expériences et observations sur l’électricité faites a Philadelphie en Amérique …, trans. Thomas-François Dalibard (2nd ed., 2 vols., Paris, 1756), I, 208–11. For the original passage see IV, 28–9.

5Ferry gives his address as “Ferry fils, rue de la Roquette près l’oratoire.” The person who delivered the letter, a M. Parraud, wrote on June 8 to remind BF of his promise to reply. APS.

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