The device is known as Ruhmkorff's coil. The first exemplar was in fact constructed in 1851 by Heinrich Daniel Ruhmkorff (1803-1877), a German precision instrument maker who began working in Paris in 1840. Following his death, his work was continued by the French engineer Jules Adrien Carpèntier (1851-1921). It is a device which, when powered by one or more piles, supplies a very high, unidirectional pulsating tension. Presented for the first time at the Paris Exposition in 1855, it was used to power discharge tubes and to produce x-rays. It consists of two superimposed coils wound on a spool of laminated soft iron, one with thick copper wire with few turns (inducer or primary coil) and the other of thin wire with many turns (induced or secondary coil). The wire is insulated with great care; it is in fact covered with silk and the spires are separated one from the other by a layer of dried, fresh shellac.
In the Ruhmkorff models, in the primary coil some tens of metres (even 50) of 2 mm wire were used, while in the secondary coil the wire was a few tenths of a millimetre in diameter and its length could reach, and even exceed, one hundred kilometres.
The specimen presented here is rather small: it is 14.5 cm long and has a diameter of 7.75 cm, while the large models measure up to 65 cm in length and 25 cm in diameter. Depending on the model, the primary coil is powered by one or more piles and the connection takes place by means of a special commutator. But the current also passes through the contact between the point of an adjustable metal screw and an flexible copper reed which, having one end fixed, acts as a spring.
The free end of the reed has a hammer of soft iron which, in the rest position, almost lies on the core.

Daguin (1878), T. III, p. 762
Du Moncel (1879), p. 247
Privat Deschanel (1869), p. 771
Turner (1983), p. 184

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