Figure 47 represents the revolving armatures and coils, with the case taken off. C and C are the two coils of insulated copper wire, surrounding two straight bars of soft iron, represented in the end view by D and D. E is the shaft. The two armatures and coils are secured to the two brass straps F, which are themselves fastened upon the shaft. The armatures are allowed to project through the straps about the sixteenth of an inch. On each end of the shaft is attached an _unitrep_, consisting of two cylindrical segments of silver, as seen at 5 and 5, figure 45; insulated from each other, and secured to a cylinder of ivory or wood, upon the shaft, so as to revolve with it. The terminations of the coils of wire upon the armatures, are soldered to the segments of silver, and as the unitrep turns, it brings opposite ends of the wires, alternately, upon the stationary wires or conductors, P and N: (in figure 46 they are represented by _a_ and _c_, and 3 and _a_.) The opposing currents of the coils, in each half revolution, are, by this contrivance, made to form one continuous current. Hence, the name _unitrep_ (to turn together.) There being two unitreps, and corresponding conducting wires, and screw cups, the induced currents from the two coils may be combined in several ways, after the manner of combining separate batteries. Let the wires below the base board be all properly connected with the receiving cups, as heretofore described. Then let the wire from 6, (represented by dots,) to _k_, be connected with the wire 9 and _m_; and also the wire 7 and _l_, with the wire 8 and 0. Let one of the united wires be connected with one wire leaving the coil of an electro magnet; and the other united wire be joined to the other wire of the electro magnet of the telegraph, or any other instrument designed to operate by a galvanic battery. When this preparation is finished, if the armatures and coils are made to revolve rapidly, a powerful current is formed in the induced coils, C and C, figure 47, capable of performing all the experiments generally made by means of the galvanic battery. Dr. Page has made a very important discovery in connection with this machine, not now to be made known; but, suffice it to say, the single machine which he has now in his possession, on Christmas day, 1844, operated Morse’s telegraph, through the circuit of 80 miles; half this circuit being wire, the other half the earth. This machine makes an electro magnet sustain 1000 pounds, and melts a platinum wire one-fortieth of an inch in diameter. _The Pole Changer._ We introduce here a description of an instrument used for reversing the direction of the galvanic current, and which is applied in the operation of several kinds of electric telegraphs. There is a variety of modes by which the same object is attained, but as this appears the most simple, we have chosen it in preference to others.