534 MAGNETISM AND ELECTRICITY loud-speakers of the dynamic type the current is sent through a coil of wire attached to a cone-shaped diaphragm. This coil surrounds a permanent magnet or an electromagnet operated by a steady current. As the current in the coil varies, it causes the coil with attached diaphragm to vibrate back and forth. This can be easily understood when one considers that the current passing through the coil sets up an alternating electromagrtetic field which interacts with the field of the magnet. The vibration of the cone sets up sound waves. Large cones operated by strong currents may set up sound waves which will enable the voice to be heard at a distance of a mile or more. Loud-speakers and electrical amplification have practically rendered obsolete the old mechanical type of amplification in phonograph machines, and they are now widely used very effectively whenever it is desired that a speaker be heard by large crowds. A combination of amplifier, loud-speaker, and microphone is called a public-address system. Microphones were formerly based on the same principle that is employed in carbon-particle telephone transmitters except that a double button, with carbon granules on each side of the diaphragm, permitted more faithful reproduction. These were too poor in response and produced an undesirable hiss that led to their abandonment. Modern studio microphones are of the condenser type, in which one "plate " of a condenser is a light metal diaphragm whose motion, caused by sound waves, alters the capacity of the condenser and thus produces oscillations in the circuit. A number of other types of microphones are also used today, including piezo-electric crystal types. Electrical Transcription Was Made Possible by the Electrical Phonograph Pickup. The electric phonograph resembles the public-address system except that an electrical phonograph pickup replaces the microphone. The needle in one type of pickup is mounted in a movable frame suspended between the poles of a permanent magnet. The needle is surrounded by a coil of wure in which an alternating current is induced as the vibration of the needle causes a variation in the strength of the magnetic field between the poles of the magnet. The needle is caused to vibrate as it follows the wavelike path on the phonograph record. The current induced in the coil is then amplified by means of amplifying-tubes until it is powerful enough to operate a loud-speaker. An electric phonograph gives a more faithful reproduction than the former mechanical amplification of the nonelectric phonographs because the vacuum-tube amplifier amplifies all frequencies within limits and uniformly within limits.
RADIO GENERATION AND RECEPTION 535 Automatic Volume Control Eliminates Fading. Modern radios which have automatic volume control prevent fading by using a portion of the radio waves to produce a control current that increases or decreases with the strength of the grid waves, which is then used to change the amplification of the receiver. Frequency Modulation Eliminates Static. Edwiji H. Armstrong invented frequency modulation, which is a system of broadcasting whereby static is largely eliminated and more exact reproduction of tonal qualities is made possible. Better reproduction is achieved by using a greater frequency range than is employed in ordinary radio broadcasting. Modulated + Unmodulated Zero + Zero Unmodulated 'Modulated Fig. 263. Amplitude modulation (AM) and frequency modulation (FM). (Courtesy of Quest Science Summary.) Modulation refers to the changes which take place in a radio wave as it reacts to the electrical impulses caused by sound waves. Radio broadcasting stations have been using amplitude modulation in which the strength, or amplitude, of the radio waves varies with the sound that they carry. In frequency modulation, on the other hand, the frequency of the radio wave is varied while its strength is kept constant. The reason why amplitude-modulation radios have so much static noise is that lightning flashes, diathermy machines, X-ray machines, neon lights, electric razors, automobile ignition systems, dial telephones, thermostats, vacuum cleaners, and similar devices produce electromagnetic waves, some of which have the same frequency as that of the broadcasting station. In the amplitude-modulation radios the only way to overcome this static is to produce such powerful waves that they will overshadow the weaker static waves. In the