Sturgeon, the parent of the electro-magnet, on learning of Henry’s discoveries and inventions, wrote: “Professor Henry has been enabled to produce a magnetic force which totally eclipses every other in the whole annals of magnetism; and no parallel is to be found since the miraculous suspension of the celebrated oriental impostor in his iron coffin.” (_Philosophical Magazine and Annals_, 1832.) The third decade was now prepared for the development of the telegraph. As to the telegraph in its broadest sense, as a means for conveying intelligence to a distance quickly and without a messenger, successful experiments of that kind have existed from the earliest times:--from the signal fires of the ancients; from the flag signals between ships at sea, introduced in the seventeenth century by the Duke of York, then Admiral of the English fleet, and afterwards James II of England; from the semaphore telegraph of M. Chappe, adopted by the French government in 1794, consisting of bars pivoted to an upright stationary post, and made to swing vertically or horizontally to indicate certain signals; and from many other forms of earlier and later days. As to electricity as an agent for the transmission of signals, the idea dates, as already stated, from the discovery of Stephen Gray in 1729, that the electrical influence could be conveyed to a distance by the means of an insulated wire. This was followed by the practical suggestions of Franklin and others. But when, as we have seen, voltaic electricity entered the field, electricity became a more powerful and tractable servant, and distant intelligent signals became one of its first labors. The second decade was also made notable by the discovery and establishment by George Simon Ohm, a German professor of Physics, of the fundamental mathematical law of electricity: It has been expressed in the following terms: (a) the current strength is equal to the electro-motive force divided by the resistance; (b) the force is equal to the current strength multiplied by the resistance; (c) the resistance is equal to the force divided by the current strength. The historical development and evolution of the telegraph may be now summarized:-- 1. The discovery of galvanic electricity by Galvani--1786-1790. 2. The galvanic or voltaic battery by Volta in 1800. 3. The galvanic influence on a magnetic needle by Romagnosi (1802) Oersted (1820). 4. The galvanometer of Schweigger, 1820--the parent of the needle system. 5. The electro-magnet by Arago and Sturgeon--1820-1825--the parent of the magnet system. Then followed in the third decade the important series of steps in the evolution, consisting of:-- _First_, and most vital, Henry’s discovery in 1829 and 1830 of the “intensity” or spool-wound magnet, and its intimate relation to the “intensity” battery, and the subordinate use of an armature as the signalling device.