Siemens and Wheatstone almost simultaneously invented so much of the dynamo as was yet incomplete. It has "cores"--the parts that answer to the legs of a horseshoe magnet--of soft iron, sometimes now even of cast iron. These, at starting, possess very little magnetism--practically none at all--yet sufficient to generate a very weak current in the coils, windings, of the armature when it begins to turn. This weak current, passing through the windings of the field magnet, makes these still stronger magnets, and the effect is to evolve a still stronger current in the armature. Soon the full effect is reached. The big iron field magnet, often weighing some thousands of pounds, is then the same as a permanent steel horseshoe magnet, which would hardly be possible at all. One who has watched the installation of a dynamo, knowing that there is nowhere near any ordinary source of electricity, and has seen its armature begin to whirl and hum, and then in a few moments the violet sparklings of the brushes and the evident presence of a powerful current of electricity, is almost justified in the common opinion that the genius of man has devised a machine to _create_ something out of nothing. It is true that a _starting_ quantity of electricity is required. It exists in almost every piece of iron. Sometimes, to hasten first action, some cells of a galvanic battery are used to pass a current through the coils of the field magnet. After the first use there is always enough magnetism remaining in them during rest or stoppage to make a dynamo efficient after a few moments operation. [Illustration: PACINOTTI'S RING-ARMATURE DYNAMO.] This is the dynamo in principle of action. The varieties in construction now in use number scores, perhaps hundreds. Some of them are monsters in size, and evolve a current that is terrific. They are all essentially the same, depending for action upon the laws illustrated in the simplest experiment in induced electricity. One of the best known of the modern machines is Edison's, represented in the picture at the head of this article. In it the field magnet--answering to the horseshoe magnet of the magneto-electric machine--is plainly distinguishable to the unskilled observer. It is not even solid, but is made of several pieces bolted together. Its legs are hollowed at the ends to admit closely the armature which turns there. There are valuable peculiarities in its construction, which, while complying in all respects with the dynamo principle, utilize those principles to the best mechanical advantage. So do others, in other respects that did not occur even to Edison, or were not adopted by him. Probably the modern dynamo is the most efficient, the most accurately measurable, the least wasteful of its power, and the most manageable, of any power-machine so far constructed by man for daily use.