Another improvement was to do away with the parchment. Graham Bell found that a sheet of tin would do very well instead of the parchment. What is called “tin” is usually in reality a sheet of iron covered on both sides with a very thin layer of metallic tin; and as the magnetism acts quite easily through tin, the tinned iron acts like a sheet of pure iron. When a sound wave reaches it, it is set vibrating; and its vibrations slightly alter the magnetism of the permanent magnet, and produce the small and rapid currents mentioned above. The whole apparatus, therefore, consists simply of two steel magnets, two discs of tin, sufficient wire to make coils round the magnets and cover the distance between the two stations, and the necessary boxes to keep the discs and magnets in the proper position. It is surely the simplest instrument ever invented to perform such a wonderful task. What is almost quite as wonderful is that this telephone is still in general use for receiving telephone messages. It says a great deal for the excellence of the instrument that thirty-three years of active search for improvements have not been able to suggest any real improvement in Graham Bell’s instrument. Bell’s receiver is still used at the present day. But his instrument is not as good for sending or transmitting speech as some other instruments which have since been invented. [Illustration: Fig. 60.—Carbon Transmitter.] [Illustration: Fig. 61.--Combined Receiver and Transmitter.] It was found that Bell’s transmitter was rather feeble as regards loudness, and an improved instrument has been invented in America since his time. It was Edison, the inventor of the phonograph, who first proposed to use carbon for sending the message. The new instrument was in some ways a revival of the idea of Reis, but instead of breaking and making the circuit, it was thought better to make the contact between the ends of the circuit weaker and stronger in succession. This is now done in an instrument called the Blake transmitter. [Illustration: Fig. 62.--Microphone.] An electric circuit containing a battery is interrupted at one point, and joined again through a little ball of platinum and a ball of carbon, such as is used in electric batteries. This contact is connected with a vibrating plate which transmits the speech. The circuit is never really broken, but is weakened and strengthened alternately by a more or less perfect contact between the two balls. The result is that human speech is conveyed with great clearness.