The stepping stone to the dynamo in its development was the _magneto-electrical machine_. This is a machine founded upon the general principle observed by Faraday in 1831 and 1832, and also by Prof. Henry about the same time, that when a magnet is made to approach a helix of insulated wire it causes a current of electricity to flow in the helix as long as the magnet advances. If the magnet is passed through the helix, the current is reversed as soon as the magnet passes the middle point. The principle is the same if the magnet be made to approach and recede from the poles of an electro-magnet having a helix wound around a soft iron core. Likewise the same result occurs if the electro-magnet with its helix is made to approach and recede from a permanent magnet, the current in the helix flowing in one direction when it approaches the permanent magnet, and in the opposite direction when leaving the said magnet. The movement of the two elements in relation to each other requires some force to overcome the repellent and attractive actions, and this force is converted into electrical energy. This is the principle of the magneto-electric machine. [Illustration: FIG. 17.--PIXII MAGNETO-ELECTRIC MACHINE, 1832.] Saxton in the United States and Pixii in France were the first to produce organized devices of this class for generating electricity from magnetism. Pixii’s machine (1832) consisted of a permanent horse-shoe magnet which was caused to revolve in proximity to an armature upon which was wound a coil of insulated wire. On March 30, 1852, Sonnenberg and Rechten obtained a United States patent, No. 8,843, for an electrical machine for killing whales, and on August 19, 1856, Shepard obtained U. S. Pat. No. 15,596 for the machine which came to be known as the “Alliance” machine. Both of these machines had permanent field magnets, and were early types of magneto-electric machines. The efficiency of these magneto-electric machines was necessarily limited to the strength of the inducing field magnets, which, being permanent magnets, were a positive and fixed factor. It was an easy step to substitute electro-magnets for permanent magnets, as the field or inducing magnets, and also to excite the (electro) field magnet by voltaic batteries, but the important step which resulted in the machine which is called the “dynamo” (from the Greek “Δυναμις”--power) was yet to come. [Illustration: FIG. 18.--HJORTH’S DYNAMO ELECTRIC MACHINE.] [Illustration: FIG. 19.--HJORTH’S DYNAMO ELECTRIC MACHINE, PLAN VIEW.]