WHEN GALENA RULED RADIO
At the dawn of commercial radio broadcasting in the 1920s, simple crystal radio receivers brought music and comedy programs, along with news and weather reports, into the homes of millions of American families. The key component in these inexpensive and often homemade receivers was the rst practically applied semiconductor material—a tiny crystal of galena (lead sulfide).
NATURAL SEMICONDUCTORS
Semiconductors are solid materials with levels of electrical conductivity between those of conductive metals like copper and non-conductive insulators such as glass. Scientists began studying semiconductors in the 1830s. In 1874, German physicist Ferdinand Braun observed that electrical resistance in certain metal-sulfide crystals varied with the direction of the current. Braun also learned that the contact point between a thin wire electrode and a galena crystal could modify electrical current and produce what he called the “point-contact electrical-rectifier effect.”
Researchers who subsequently studied pyrite (iron disulfide), bornite and chalcopyrite (both copper iron sulfides), molybdenite (molybdenum disulfide), zincite (zinc oxide), and other mineral semiconductors eventually concluded that galena had the most desirable semiconductor properties.
GALENA: THE FIRST SEMICONDUCTOR
e galena crystal lattice is built of a repetitive sequence of lead and sulfur atoms. Pure galena crystals consisting of just lead and sulfur function only as weak electrical conductors. But galena almost always contains traces of such metals as copper and silver that create an excess or defficiency of free electrons at regular lattice sites. ese sites act as “electron traps” that allow electrons to move in one direction only.
Not all galena is a good semiconductor. The most desirable semiconductor crystals have specific levels of metal impurities and a crystallographic structure that enables surface areas to function as electrical-rectifier contact points.
GALENA IN RADIO
In 1900, American inventor Greenleaf Whittier Pickard first used galena crystals in simple radio receivers. These so-called
“crystal sets” consisted of a long wire antenna, a tuning circuit with two concentrically positioned wire coils, a carbon-particle earphone and a pea-sized crystal of galena.
The galena crystal performed two functions: it rst converted alternating current into direct current; then it detected and separated the audio-wave component from the carrier-wave component of a radio-wave-induced microcurrent. Crystal set operators simply moved a exible, pointed copper wire called a “cat’s whisker” across the galena crystal to locate the site that maximized the point-contact recti er e ect and the loudness of the audible earphone signal.
CRYSTAL SETS
When commercial radio broadcasting began, the cost of battery-powered home radio receivers that employed crude vacuum-recti er tubes and ampli ers was prohibitive. e only a ordable alternative was the crystal set that could be bought ready-to-use for only a few dollars or easily built for even less from inexpensive components.
The galena crystal in a crystal set was a small, irregular fragment of a larger crystalline mass. Weighing only a few grams, a semiconductor galena crystal cost about ve cents and was readily available at scienti c-supply houses and even many hardware stores. e cost was not that of the galena itself, which was mass-mined as the primary ore of lead, but from the expense of testing each crystal to make sure it could function as an electrical semiconductor.
By the late 1920s, an estimated 40 million crystal sets, by far the most popular type of radio receiver, were in use worldwide. But their heyday was cut short by the appearance of improved, mass-produced vacuum-recti er tubes that sharply reduced the cost of battery-powered, ampli ed radio receivers.
While crystal sets were technically obsolete by 1930, they nevertheless remained popular, especially in rural regions that were still awaiting national electri cation. Crystal sets even served as emergency radio receivers on many naval and merchant ships during World War II.
GALENA’S RADIO LEGACY
e 1950s brought a huge resurgence of interest in crystal sets among hobbyists and as an educational tool. Crystal radio sets remain available today as both build-it-yourself kits and ready-to-operate receivers. Now, however, the “crystal” is a synthetic germanium diode—a far more e ective semiconductor than galena.
Radios today are vastly improved and far more complex devices that utilize diodes, digital tuners, dipole antennas, integrated circuits and wireless connectors. But it is interesting to remember that commercial radio broadcasting got its start with millions of inexpensive crystal set receivers, each built around a tiny crystal of galena.