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　　Magnets store much of the world’s information: data on computer disks, entertainment on video and audiotapes, messages on telephone answering machines and account information on the coated stripes of ATM and credit cards. All these different media preserve words, numbers, images and sounds as invisible patterns of north and south poles. The technology is magnetic recording, which celebrates its centennial this year.

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　　With less notoriety, the magnetic memories of computers have been quietly keeping track of people, goods and money for many years. In the information age, they have served as the warehouses of electronic knowledge, storing scientic, medical, industrial and financial data. Although other media, such as film, paper and CD-ROMs, are also widely used, magnetic recording boasts one huge advantage that distinguishes it from most competing technologies: it can easily record or erase information (as demonstrated by secretary Rose Mary Woods and the infamous gap in the Nixon tapes).

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　　People have known for some time that certain matrials "remember" the direction in which they were magnetized. As early as 200 B. C., Chinese magicians of the emperor's court made compasses out lodestones, iron-rich rocks that are strongly magnetic.

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　　An employee of the Copenhagen Telephone Company, Poulsen thought that people would want a device to record telephone messages. He demonstrated the concept of his invention to his friends with a steel piano wire stretched across his laboratory. Sliding an electromagnet along the wire, Poulsen began yelling into a telephone mouthpiece connected into the electromagnet to generate a magnetic field that changed with the volume, pitch and other characteristics of his voice. The varying field was then imprinted along the steel wire.

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　　Poulsen quickly refined his invention--in one variation, the steel wire was wrapped around a cylinder--and applied for patents in various countries. The initial response, though, was uniformly negative. In the U.S. a patent examiner wrote that the telegraphone would fail because its claim were "contrary to all known laws of magnetism”. Apparently, the laws of magnetism were not all known in those days, because Poulsen' s invention did indeed work.

　　A series of demonstrations in Europe soon proved the device. At the Paris Exposition in 1900, the telegraphone was a spectacular success, impressing many of the visitors, among them Emperor Franz Joseph of Austria, who recorded a message that remains the oldest magnetic recording in existence today.

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　　Even so, the telegraphone appeared to have a technical edge. An article in Scientific American in 1900 stated that the sound produced by the telegraphone "is very distinct and is entirely free from the disagreeable scratching noises generally heard in the phone graph.'' Nevertheless, another half century would pass before magnetic recording found widespread application.

　　A. After Poulsen reached the end of the wire, he returned to the starting point and replaced the mouthpiece with a receiver. When one of his friends slid the electromagnet along the wire, the device worked in reverse, first detecting the field in the wire, converting that time-varying information into an electric signal (via electromagnetic induction) and turning the current back into sound. Poulsen's friend could hear the inventor's voice faintly in the receiver!

　　B. But magnetic recording, introduced 100 years ago, was an invention that languished for decades. Some of the underlying physics was unknown, suitable applications were not quite ready, and business and political obstacles conspires against early adoption. In its second haft-century, however, the technology quickly became an invaluable tool of modem society.

　　C. Of course, Poulsen was not the first to record and reproduce the human voice. More than two decades earlier, Thomas Edison had patented the phonograph, which originally recorded sound with grooves cut in tin foil wrapped on a cylinder. Later, wax cylinders and then plastic disks were used, and by the time Poulsen' s invention came to the world's attention, the phonograph was already an established product.

　　D. In recent decades magnetic memories have had a profound influence on society. In the 195s the Watergate tapes from the Oval Office provided the 'smoking gun' that forced President Richard Nixon to resign. This year audiotapes that disclosed an improper relationship between President Bill Clinton and a former White House intern led to another executive scandal.

　　E. Unfortunately, refinement of the telegraphone was hampered by a meager under standing of the basic physical phenomenon on which the device depended. The conversion of sound into electricity and then into a magnetic field was fairly well under stood from earlier experience with the telephone. But the mechanism by the history of those magnetic fields could be stored in a recording medium, such as a steel wire,was something of a mystery at the time. Another limitation was the time. Another limitation was the Iow level of playback sound.

　　F. But it was not until the 1800s, when scientists began to uncover the link between electricity and magnetism, that magnets began to play a key role in such notable inventions as telegraphs, telephones, generators, transformers and motors. Then, near the end of the 19th century, a magnetic material was used to record and reproduce the human voice. The device, called a telegraphone, was patented in 1898 by Valde mar Poulsen, a Danish engineer.