There are two principal kinds of dynamos--those producing the alternating currents, and those producing the continuous current. In the first the current alternates in direction, or is composed of an infinite number of impulses of opposite polarity: one polarity when a section of the armature coil is approaching a north field magnet pole or receding from a south pole, and the other polarity when receding from a north field magnet pole and approaching a south pole. In the continuous current machine, the commutator and brushes are so arranged as to take up all the impulses of the same polarity and conduct them away by one brush, and gathering all the impulses of the opposite polarity and conducting them away by another brush. Thus the current of each brush, in the continuous current machine, is always of the same polarity, and the polarity of one being always positive, and that of the other negative, the current flows continuously in the same direction. A third species of dynamo is the pulsatory, in which the current flow is invariable in direction, but proceeds in waves. [Illustration: FIG. 21.--ARMATURE OF BIPOLAR DYNAMO.] A change in the character of the current generated by the dynamo is made by what is known as the “transformer,” in which the principle of the induction coil is made available. In this way, for instance, the high potential currents generated by the powerful water wheels at Niagara Falls are taken twenty miles to Buffalo, and are there transformed into other currents of lower potential, suited to incandescent lighting and other various uses. A similar scheme is in process of fulfillment in the establishment of a water power electric plant near Conowingo, Maryland, on the Susquehanna River, to furnish electrical power to Baltimore, Wilmington and Philadelphia. An important development in electrical generation and transmission is to be found in what is known as the _polyphase_, _multiphase_, or _rotating_ current, pioneer patents for which were granted to Tesla May 1, 1888, Nos. 381,968, 381,969, 382,279, 382,280, 382,281 and 382,282. Realizing the possibilities of the dynamo, the Legislature of New York in 1888 passed a law, which went into effect in 1889, in that State, substituting death by electricity for the hangman’s noose. The criminal is strapped in the chair, seen in Fig. 22, one terminal of the wire from the dynamo is strapped upon his forehead, and the other to anklets on his legs, and like a flash of lightning the deadly energy of the dynamo performs its work.