This task, as above illustrated, we perform by sending to a receiving pencil a current varying in strength between limits which correspond to the variations in length of our transmitted lines. The strength of this current, say 0.429 volt, decides where a mark will begin; the strength of that current in rising to say 27.5 volts, decides where that mark will end. To vary the strength of the current as desired we employ a square rod of aluminium, tightly covered with a thin copper wire insulated by silk wrapping. We place this rod beside our tablet, and scrape from its innermost surface the silk covering so as to leave the wire bare, while between its strands the silk remains intact as an effective insulation. Our rod is now a rheostat, whose use we shall presently discover. We are wont to think of copper as a good conductor, and so it is. Used in stout bars or thick wires it exerts but little resistance to an electric current, but when we employ a wire of but 1/200 of an inch in diameter, about the thickness of the paper on which this is printed, the narrowness of path reduces the pressure of a current so much that in the course of 375 feet it falls to one eighth. In like manner a glass tube of minute diameter might receive at one end water under extreme pressure, and at a yard distance send out a mere dribble. The copper wire of our square rod, or rheostat, is so thin that when connected at K with a source of 110-volt electricity, at V this voltage, or pressure, has sunk to but one twentieth of a volt. Let us suppose our rheostat at V connected with a circuit extended to the receiving station. A wire, kept in this circuit, and moving up and down with our pencil, in a line always parallel with the side of our tablet, sends to the receiving station a current constantly varying in its pressure. As the wire passes from S to M the transmitted current rises from 0.429 to 27.5 volts. At the receiving station we provide means whereby the current arriving at a voltage of 0.429 and rising to 27.5 will mark a vertical line the length of S M. A simple device for this purpose consists in a hollow coil of copper wire, or a solenoid, as electricians call it, through which circulates the arriving current, the coil being free to be drawn as a shell over a cylindrical electro-magnet. The degree to which such a coil, duly attached to a retractile spring, is drawn over a suitable electro-magnet, depends upon the strength of the current circulating in the coil. In the simple instrument we are using let us assume that when a current of 110 volts comes in, the coil moves to K, the end of its path; that when a current of 6.875 volts arrives, the coil moves to O; the receiving coil and the sending rheostat being marked with the same divisions. Our receiving coil actuates a pencil which accordingly marks a line of the same length and direction as that set down on the tablet of the sending instrument.