In order to make the marks on the paper of different lengths, there is a contrivance for regulating the length of time that the current shall pass through the coiled wire. This contrivance, called the _signal key_ is represented in Fig. 289. N and P are two strips of brass connected with the two wires R and M, of which M comes from the battery. The end of the strip N is raised a little above the end of P. So long as they do not touch the circuit is not complete, and no electricity passes. But if the operator press N down upon P, the circuit is established, and the electricity passes to the station with which he is in communication, and there acts upon the apparatus seen in Fig. 287. Now the longer the finger presses down N upon P the longer will be the mark on the paper at the distant station. An operator then at New York, for example, controls by this key the length of the marks made on the paper in New Haven or any other place with which he is communicating. You can see then very readily how a telegraphic alphabet can be constructed by combinations of marks of different lengths agreed upon to represent different letters and numerals. I give the alphabet used in connection with Morse's Telegraph: A -- ---- B ---- -- -- -- C -- -- -- D ---- -- -- E -- F -- ---- -- G ---- ---- -- H -- -- -- -- I -- -- J ---- -- ---- -- K ---- -- ---- L -------- M ---- ---- N ---- -- O -- -- P -- -- -- -- -- Q -- -- ---- -- R -- -- -- S -- -- -- T ---- U -- -- ---- V -- -- -- ---- W -- ---- ---- X -- ---- -- -- Y -- -- -- -- Z -- -- -- -- Numerals. 1 -- ---- ---- -- 2 -- -- ---- -- -- 3 -- -- -- ---- -- 4 -- -- -- -- ---- 5 ---- ---- ---- 6 -- -- -- -- -- -- 7 ---- ---- -- -- 8 ---- -- -- -- -- 9 ---- -- -- ---- 0 --------------- One of the most singular and interesting things in the operation of the telegraph remains to be noticed. In order to have the electricity work it is necessary to have the connection between the poles of the battery at the point where the effect is to be produced. You see this in the experiments represented in Figs. 286 and 287. The same is true of the electro-magnet of the telegraph. This being so, it was thought at first that it was necessary to have two wires connecting two communicating stations; but it was found that only one wire was needed, the earth itself answering the same purpose as another wire. To make the communication through the earth effectual, there is at each station a plate of metal, having a surface of several square feet, buried in the ground, with a wire running up to the machine. QUESTIONS.