In continuing the history of the invention, as regards the different modes by which communications are transmitted along the insulated wires, the next telegraphs that deserve notice are those of Dr. Steinheil, which became known also in 1837. One of his telegraphs made the signals by sounds, produced by magnetic needles striking, when deflected, against bells of different tones. By another telegraph of his invention the symbols where marked upon paper by small tubes holding ink, fixed to the needles. In this manner the letters of the alphabet were indicated by dots upon a strip of paper, kept slowly moving by clock mechanism. This telegraph could be worked by a single circuit; and it appears that Dr. Steinheil was the first who discovered, or at least who practically applied, the conducting power of the earth for the return current. Each circuit, therefore, consisted of only a single wire; the wire that had been previously used to complete the circuit being superseded by burying in the earth, at each terminus, a small copper plate. Dr. Steinheil also introduced the use of galvanized iron wire. An electric telegraph of this construction was put into operation at Munich, through a distance of 12 miles. [Illustration] In the following year Messrs. Cooke and Wheatstone so far simplified the arrangements of their needle telegraph as to make all the requisite signals with two needles. With a single combined battery and two wires six primary signals are thus obtained; and by repeating the deflections and combining the action of the two needles, all the letters can be readily and quickly indicated. A single needle instrument was invented by Messrs. Cooke and Wheatstone, but as there are only two primary signals, one to the right and one to the left, the deflections are necessarily repeated more frequently, and the transmission is consequently more slow. The accompanying diagram represents the alphabet of the single needle instrument. The deflections for each letter commence in the direction of the short marks, and end with the long ones. Thus, to indicate the letter _R_, the needle is first deflected once to the left and then once to the right; and the letter _D_ has the deflections reversed, beginning with one to the right and ending with one to the left. In no instance does it require more than four deflections to indicate a single letter, yet the transmission with the double needle is found so much quicker that the single needle instrument is only rarely used.