Man's physical universe

xanabras

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.

More magazines by this user
Similar magazines