The credit of the successful solution of this great problem is due to an American of the name of Graham Bell. As is very often the case, the successful instrument was extremely simple, so simple indeed that people were disappointed on seeing it. It could be put together by a schoolboy, and yet it was able to talk. From the very first, Bell had seen the correct principle which must govern the construction of a good telephone. The fault of Reis’s telephone was that it could not follow the delicate wavelets of sound which constitute human speech. It was necessary to make an instrument which should be extremely sensitive and delicate, as much so as the human ear, which is saying a great deal. In order to do this Bell used Faraday’s induction currents. We have seen already that when Faraday pulled off the cap of the electromagnet a momentary current was induced in the coil of wire. Small currents could be induced by simply shaking the cap to and fro. Now, Bell had the idea of mounting a small piece of iron in front of the bar of an electromagnet. The ends of the coil of wire of the electromagnet were pulled out to a very long distance and passed round another electromagnet. The small piece of iron was stuck upon a piece of parchment, which was stretched on a drum in front of the first electromagnet. On speaking into this drum the parchment was set into vibration, and with it the small piece of iron. This produced slight and very rapid currents in the coil of the electromagnet, and these were transmitted along the wire to the other electromagnet. [Illustration: Fig. 58.--Graham Bell, inventor of the Telephone.] Another parchment with a similar small piece of iron was mounted on a drum in front of the second electromagnet, and it was found that the second drum reproduced exactly all that was spoken into the first drum. For the small currents arriving at the second electromagnet made its magnetism stronger and weaker in rapid succession, and attracted the iron sometimes more and sometimes less. Every motion of the first parchment, therefore, was rendered faithfully by the second parchment, and the vibrations of the air inside the first drum were reproduced accurately in the second drum. That was all that was necessary for the complete transmission of human speech. But this apparatus was still capable of considerable improvement. In the first place, we know that an electromagnet loses all its magnetism when the current in its coil is stopped. But Graham Bell found that it was not at all necessary to use an electromagnet. [Illustration: Fig. 59.--Bell’s Magnetic Telephone.] Instead of a piece of soft iron, he found he could use a similar piece of steel which had been magnetised strongly. Such a piece of steel is called a permanent magnet. The little horseshoe magnets with the red paint on them that are sold in shops are such permanent magnets. This discovery simplified matters greatly, as it was no longer necessary to use a battery at all.