The secret of these phenomena was found to lie in the fact that a magnet, or a conductor carrying a current, was the centre of a field of force of very considerable extent. Such a field of force can be familiarly shown by placing a piece of glass or white paper sprinkled with fine iron filings upon the poles of a magnet. The filings will be drawn into concentric circles, whose extent measures the magnet’s field of force. So also the extent of the field of force surrounding a conductor carrying a current may be familiarly shown. In these instances the filings brought within the fields of force are magnetized. So would any other conducting substance be, and would become capable of carrying away as an independent current that which had been induced in it. Here we have the essential principle of the modern dynamo-electric machine, commonly called simply dynamo. Faraday actually constructed a dynamo, which answered very well for his experiments, but failed in commercial results because the only source of energy he could draw upon in his time was that supplied by the rather costly voltaic cells. During Faraday’s time and subsequently, electricians in Europe and the United States were active in formulating further laws relative to the nature, strength, and control of electrical currents, and each year was one of preparation for the coming leap of electrical science into the vast realm of commercial convenience and profit. III. THE TELEGRAPH. From the date of the discovery that electricity could be conducted to a distance, dreams were indulged that it could be made a means of communicating intelligence. In the eighteenth century, many attempts were made to carry intelligent signals over electric wires. Some of these were quite ingenious, but in the end failures, because the old-fashioned frictional electricity was the only kind then known and employed. Even after the discovery of the voltaic cell or battery, which afforded an ample supply of chemical electricity to operate a telegraphic apparatus, the time was not ripe for successful telegraphy, for up till 1830 no battery had been produced that was sufficiently constant in its operation to supply the kind of current required. For feasible telegraphy, two important steps were yet necessary. One was the discovery of the electro-magnet, 1825–30. The other was the discovery of the Daniell’s battery or cell, in 1836, by means of which a constant electric current could be sustained for a long time. [Illustration: DANIELL’S CELLS.]