As early as 1910, the three basic components and the instructions needed to construct a crystal receiver were available from mail-order electrical supply houses. All one needed was a spool of wire, a crystal detector, and earphones. The crystal detector consisted of a small piece of galena mounted in a lead base, approximately 1/4 inch in diameter, and electrically connected to a terminal. A tiny wire, called a "cat's whisker," made contact with the exposed top of the galena, and its small moveable handle allowed the listener to find the spot on the galena where the radio signal was the loudest. The cat's whisker was connected to a second terminal. One of these terminals was connected to the tuning coil.

     A tuning coil was made by winding several hundred turns of thin, insulated wire around an empty oatmeal box or similar cylindrical object. A sliding piece of metal was positioned to move across the exposed coil windings for precise tuning. To the other terminal of the tuning coil a long wire, called the antenna, was connected and strung to a tree in the backyard, the goal being to get it as high as possible. The second terminal of the galena crystal/cat's whisker combination was connected to one wire of a headset or single earphone. The other earphone wire was connected to a ground, usually a metal stake driven into the earth. Sometimes a fourth component, a fixed or variable capacitor, was added to the circuit.

     To understand what the crystal receiver meant to the early science of radio, it is necessary to look at the available wireless detector technology in the early years of the 20th century. In Marconi's 1900 wireless, the receiving device used to translate the dots and dashes of his spark transmitter was a coherer, a small rube containing iron filings that closed like a switch when receiving the electromagnetic pulses of the Morse code. Each time the filings cohered, or caused the circuit to close, current from an in-series battery flowed; then, either a buzzer sounded, a telephone receiver clicked, or an inking device recorded a coded symbolic component of the message, a dot or dash. Then a small hammer would rap the filings apart, and the entire process began again to detect the next dot or dash. The coherer could only indicate to a radio operator if a spark signal was present. Such a system might receive five or ten words per minute and was unreliable. And although the coherer was a satisfactory receiver as long as the transmitter was of the spark-gap type, it would not work with the continuous-wave and voice-transmitting systems that quickly replaced the spark. The mechanical coherer did not allow a receiver to "hear" audio, obviously a serious technical impediment to the development of wireless telephone and r,1dio broadcasting.

     Between the crude mechanical coherer and the discovery of the detecting properties of the crystal, several intermediate systems of detecting were invented and used by two of the leading early radiotelephone inventors. Between 1900 and 1901,Reginald Fessenden's Liquid Barretter and Lee de For­ est's similar Electrolytic Detector were able to detect both continuous-wave code transmission and audio. These were less reliable than the crystal detector that followed, but they did allow radio operators to hear the human voice. By 1906 de Forest was advertising a radiotelephone system with his vacuum tube, the Audion, as the detector. Whether liquid, crystal, or vacuum tube, this new generation of non-mechanical detectors that converted or rectified radio frequency into audio frequency really opened the door for the development of the radiotelephone.

     When licensed radio for the public was introduced in 1920, it was believed that the financial basis for the new commercial radio service would derive from sales of manufactured radio sets. Large companies such as Westinghouse and General Electric introduced home radios, the most popular of which was a crystal receiver in a wood box with earphones and instructions, called the Radiola 1 and the Aeriola Jr. The vacuum tube detector, as pioneered and used by de Forest r 5 years earlier, was still too expensive for most families, but there were higher­ priced radios available that used the crystal as a detector but added a vacuum tube as an amplifier to increase the volume. By the mid-192os, better programming caused a demand for radios that would play loud enough to drive a horn speaker, and manufacturers introduced radios that used vacuum tubes for both detector and amplifier.

The vacuum tube remained the technology of choice for detecting radio signals until the transistor finally replaced it in the 1960s. And what happened to the crystal receiver? It is still the entry-level radio technology of choice. Its components, in the form of kits, are readily available today. It is almost a rite of passage for a young boy or girl to build a crystal set, and technical museums still offer Saturday morning classes where parents and their children can learn to construct a crystal receiver. There is still a thrill from building your own radio, one that seemingly works by magic, using no batteries, no electricity, one that pulls faint programs from a local AM station, experiencing what your great-grandparents did almost a century ago.