the dynamo, in which the principle of action is simply reversed; electric energy delivered from the lines becomes again mechanical motion or power. Soon after Faraday’s discoveries in magneto-electricity attempts were made to construct generators of electricity from power. But the machines were small, crude, and imperfect, and the results necessarily meagre. Pixii, in Paris, one year after Faraday’s discovery was announced, made a machine which embodied in its construction a simple commutator for giving the currents a single direction of flow. This is the prototype of the commutators now found on what are called continuous-current dynamos. After Pixii followed Saxton, Clarke, Wheatstone and Cooke, Estohrer, and others, but not until 1854 was any very notable improvement made or suggested. In that year Soren Hjorth, of Copenhagen, described in a patent specification the principle of causing the electric currents generated to traverse coils of wire so disposed as to reinforce the magnetic field of the machine itself. A year subsequently the same idea was again more clearly set out by Hjorth. This is the principle of the modern self-exciting dynamo, the field magnets of which, very weak at the start, are built up or strengthened by the currents from the armature or revolving part of the machine in which power is consumed to produce electricity. In 1856 Dr. Werner Siemens, of Berlin, well known as a great pioneer in the electric arts, brought out the Siemens armature, an innovation more valuable than any other made up to that time. This was subsequently used in the powerful machines of Wilde and Ladd. It still survives in magneto call-bell apparatus for such work as telephone signalling, in exploders for mines and blasting, and in the simpler types of electroplating dynamos. The decade between 1860 and 1870 opened a new era in the construction and working of dynamo machines and motors. It is notable for two advances of very great value and importance. Dr. Paccinotti, of Florence, in 1860, described a machine by which true continuous currents resembling battery currents could be obtained. Up to that time machines gave either rapidly alternating or fluctuating currents, not steady currents in one direction. The Paccinotti construction, in modified forms, is now almost universally employed in dynamo machines, and even where the form is now quite different the Paccinotti type has been at least the forerunner, and has undergone modifications to suit special ends in view. Briefly, Paccinotti made his armature of a ring of iron with iron projections between which the coils of insulated wire were wound. Although full descriptions of Paccinotti’s ring armature and commutator were given out in 1864, his work attracted but little attention until Gramme, in Paris, about 1870, brought out the relatively perfect Gramme machine. In the mean time the other great development of the decade took place.