This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Different machines of this kind have been devised by different persons. Among them probably those of Clark, Dujardin, and the Messieurs Breton are best known. The current is broken in these instruments by a rotary movement given to the armature of soft iron, by which it is alternately brought into and removed from apposition with the magnet; but the coil of insulated wire, called the intensity coil, is in some of them placed around the armature, and in others around the magnet. This difference is of no great account, as the armature always becomes temporarily a magnet when in connection with the proper magnet, and consequently induces a current in the coil around it. But M. Duchenne, considering none of them capable of effecting all the objects attainable by a perfect machine of this order, endeavoured, by combining the best parts of those in use with improvements of his own, to make an instrument approaching nearer to his conception of what is desirable than any one hitherto invented. As this instrument, at the time when the first edition of this work was published, was, I believe, quite unknown in this country, I gave in a note a figure of it copied from one in M. Du-chenne's treatise, with his explanation of its construction and principles. This description, as no longer necessary, has been omitted in the second and present editions. I would simply state, in relation to the instrument, that the chief additions made by M. Duchenne were a graduator of the currents, by which the intensity may within certain limits be increased or diminished at pleasure, and a second superimposed coil of insulated wire, much finer than the first, by which the intensity is greatly increased, and which, though used in the volta-electric machines, had not previously been applied to the electro-magnetic.