Moving magnet cartridge

A magnetic cartridge, more commonly called a phonograph cartridge or phono cartridge or (colloquially) a pickup, is an electromechanical transducer used in the playback of analog sound recordings called records on a record player, now commonly called a turntable because of its most prominent component but formally known as a phonograph in the US and a gramophone in the UK.

The cartridge contains a removable or permanently mounted stylus, the tip - usually a gemstone like diamond or sapphire - of which makes physical contact with the record's groove. In popular usage and in disc jockey jargon, the stylus, and sometimes the entire cartridge, is often called the needle. As the stylus tracks the serrated groove, it vibrates a cantilever on which is mounted a permanent magnet which moves between the magnetic fields of sets of electromagnetic coils in the cartridge (or vise-versa: the coils are mounted on the cantilever, and the magnets are in the cartridge). The shifting magnetic fields generate an electrical current in the coils. The electrical signal generated by the cartridge can be amplified and then converted into sound by a loudspeaker.

In the early 20th century, the magnetic cartridge mostly displaced the older ceramic cartridges with single use steel needles that relied solely on mechanical transduction including pickup and a horn, to amplify sound.

The first commercially successful type of electrical phonograph pickup was introduced in 1925. Although electromagnetic, its resemblance to later magnetic cartridges is remote: it contained a bulky horseshoe magnet and employed the same imprecisely mass-produced single-use steel needles which had been standard since the first crude disc record players appeared in the 1890s. Its tracking weight was specified in ounces, not grams. This early type of magnetic pickup completely dominated the market well into the 1930s, but by the end of that decade it had been superseded by a comparatively lightweight piezoelectric crystal pickup type. The use of short-lived disposable metal needles remained standard. During the years of affluence and long-deferred consumer demand immediately following World War II, as old record players with very heavy pickups were replaced, precision-ground and conveniently long-lasting stylus tips made of sapphire or the exotic hard metal osmium were increasingly popular. However, records made for home use still played at 78 rpm and most of them were still made of the same old abrasive shellac compound formulated to rapidly wear down the points of steel needles to fit the groove.

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