Viiin1tagile 11Paidilic The saga of the Crystal Set Recently I was taken to task because, in its six years, this 'Vintage Radio' column has not yet featured crystal sets. The point has been taken, and this month we cover some aspects of this classic and in its time, most important receiver. The basic crystal receiver comprises laity. Crystal sets were inexpensive, semiconductors. Braun reported that only three components: an inductor, a simple to make, cost nothing to run and various combinations of metals with ox- diode and a headphone -- although to be what is more, they worked. ides or sulphides, with small contact ar- completely accurate, there are also two There is no receiver simpler than the eas, displayed unilateral electrical virtual capacitors formed by stray ca- basic crystal set, yet an endless series of Conduction characteristics. pacitance of the aerial and the headphone circuit variations appeared in radio and As with many early discoveries, here leads. It would be impossible to create a hobby publications and were eagerly as- was a solution waiting for a problem. simpler receiver than the elementary similated — previous models torn apart Braun went on to other work, including crystal set, yet entire books have been and the new variations tried. Many cathode rays, and it was a further 25 written about it. What other type of re- young experimenters got 'bitten with the years, in 1901, before he conducted ex- ceiver has remained basically unchanged radio virus' as a result, and went on to periments into the use of crystals for ra- for 80 years? make radio their careers. dio detection. He worked with galena In the period from the advent of broad- Like much electronics technology, the (lead sulphide) and iron pyrites, but it casting to the arrival of the cheap transis- origins of research in the electrical char- was psilomelane, a mineral containing tor receiver, making crystal sets was one acteristics of crystals go back to the 19th manganese, with a sensitivity equal to of the traditional childhood activities. century. The earliest work of which I can that of the electrolytic detector, that he There were several reasons for its popu- find any record was by Ferdinand Braun considered showed the greatest promise. in Germany during 1874, in what could Braun reported his findings to Tele- be regarded as the first research into funken, but there seems to be no record Wireless Apparatus From a Contact Stud to a 16" Spark Coil can be obtained from us. eit vs r i. oErEC "r0 MIICNETIC DETECTOR. aniasrmaLe coNornSeR FMCb ComoEWER (.3 airrr.cmy 3. • 5 aTen 17.3-4417 INCTIMININ COnWfMe ZINCITE-BORNITE DE'T'ECTOR, Mounted on Solid Ebonite, High-Class in every detail, will withstand shock or Mui. riot. TUNER vibration. 11/6 each, post free. Other types from 4/ 6 WRITE FOR CATALOGUE— pft TEL.EP.,,E TR WISrORMER (Pinto ed I TM F. L MITCHELL & Co., Ltd., CP9 LOW RESISTHNCE rearPHoNES TELEPHONE 188, RYE LANE, PECKHAM, S.E. CON0CmSER CHHNOE-Ovell SWITCH KEr London Readers should visit ou r JHowr-C.Rewerama Premises. 44-- corrromrs Fig.1: A crystal detector advertised in Fig.2(a): The circuit of a 1915 shipboard Marconi receiver installation, providing the Marconi 'Yearbook of Wireless the choice of a magnetic detector emaggie') or a type 20 crystal set. Note the Telegraphy & Telephony' for 1914. battery and rheostat for biasing the crystal. 98 ELECTRONICS Australia, July 1994 • R firr, r 1 A.. B0- 150 ,.,,,,, 0: RR rye c 2. A - 150-16o.,,,,. e x, K , n' a, - o - .,..2 ,21 .1C. ... , p ..- .1035 ,". riBI , 4 e Roric a 3 A . 1600 - 2c,,,,,,, ,, es • : : :... ... .... •15,qr. I • • - - ANTIQUE. E LEL, 1-IONIC P17 ,G E 4 A - loco - 26vo -,,-,.. Fig.3: The British GEC variometer • tuned Gecophone crystal set Nol of C; ... Fig.4: In 1919, US firm Quaker Oats f 1922, in a dovetailed mahogany case. .., o ri, ., - a- offered this novel crystal radio, based Ga, ... • Many sets of this era were similar in • on one of its cardboard oats cartons, appearance to scientific instruments. for $1 plus two carton labels. By 1921, Fig.2(b): The various configurations of they had issued 250,000 sets. It was the versatile Marconi multiple tuner — featured on the cover of Antique much more complex than the domestic Electronic Supply's 1991 catalog, crystal sets of a decade later. reproduced here. of that company or anyone else making encouraged widespread research into practical application of his work. crystal rectification, and in the period up The noted Indian scientist Sir Jagadis to World War I a large variety of exam- Chunder Bose, also in 1901, patented a ples appeared. The more popular detec- detector of electromagnetic radiations, tors incorporated molybdenite, galena, including Hertzian waves, which used a bornite, zincite, iron sulphide and tellu- pair of galena crystals in contact. Bose rium. Some of these depended on metal- considered that his detector worked as a lic contact, whereas others used two variable resistance self-resetting coherer, different crystals in contact. but no further development seems to Some combinations were sold under have followed. trade names. These included the Perikon (chalcopyrites and zincite), Pyron (iron Various detectors pyrites and silicon) and Bronc Cell (tel- Meanwhile, radio communication had lurium and graphite). Fig.1 illustrates a developed into a commercial enterprise detector assembly using a zin- depending for reception on magnetic and cite/bornite combination. electrolytic detectors, and coherers. Often, these crystal detector assem- Fleming's diodes, an adaptation of the blies were switchable. One gets the im- 1884 patented Edison Effect lamp, were pression that there was a suspicion of used by the Marconi Company as detec- reliability about crystals. It was common tors from 1905. to mount two or more crystals in a turret, The first practical application of a so that rapid changes could be made. In crystal as a detector has been credited to other cases, the tuner could be switched one of Lee De Forest's employees, a re- to either a magnetic detector or a crystal. tired army general, H.H. Dunwoody. Fig.&: This 1924 novelty model, known A complete 1915 shipboard receiver cir- This was in 1906, the same year that De as 'Uncle Tom, was made of cuit with this feature is shown in Fig.2. Forest himself first experimented with Staffordshire china with the coil Rather than rectifying more or less the triode valve. wound around the top hat. evenly over their entire surface, a few Dunwoody had been experimenting crystals were found to have sensitive with silicon carbide, or carborundum, vice; he found too that a polarising bat- spots that worked best when touched which is made by heating a mixture of tery increased sensitivity considerably. lightly with a piece of fine wire. This carbon and silica with an electric arc, and By applying a bias voltage, the forward piece of wire came, of course, to be whose crystals approach diamond in conduction point of the crystal was called a `catswhisker' and one of the hardness. He developed his detector to brought nearer to zero voltage. We see most sensitive crystals was a piece of the point where the crystal could be this same characteristic today in the for- galena, a common lead ore. One source clamped between steel electrodes, creat- ward voltage drop of silicon diodes. of high grade crystals was the American ing an extremely rugged and stable de- Carborundum's success seems to have town in Northwestern Illinois actually ELECTRONICS Australia, July 1994 99 Antenna The situation was different here and in VINTAGE RADIO America. Income was higher, and a fair Detector percentage of the community was remote called Galena, where at one time, a thriv- from transmitters. Even so, both com- ing mail order business was carried on mercially made and homebuilt crystal, with radio enthusiasts. sets were popular, a remarkable Ameri- Unlike carborundum, the galena crys- can example being the Quaker Oats pro- tal had the advantage of not requiring a motional model featured in Fig.4. biasing voltage to increase its sensitivity. Locally, firms like Emmco and Radiokes Biasing added the complication of a bat- manufactured components to meet the tery and rheostat, and the operator hav- needs of enthusiasts, and Wireless Weekly ing to remember to switch the current off provided them with plenty of projects. after use. Only so much could be done to im- The modern semiconductor diode is a Fig.6: The fundamental circuit used in prove a simple crystal set, but one way of direct descendent of these detectors, and the five crystal sets illustrated. improving reception was to add valves. the point contact germanium diode can Although as simple as you could get, For the Brownie user, a matching two- be seen through a magnifying glass to be its weakness is lack of selectivity due stage amplifier could be attached to pro- fundamentally a catswhisker in contact to heavy damping of the tuned circuit. vide loudspeaker reception. In many with a tiny piece of crystal. cases, a crystal detector was used in con- A crystal set's performance is vitally junction with valve RF and AF amplifi- Unsuitable at sea dependent on its tuning system. The ers, and reflexing was common. Despite its good performance with complex Marconi tuner in Fig.2 is capa- A different approach was the 'electro- weak signals, the galena crystal was not ble of superior results, but would have mechanical relay', in which a headphone popular in marine service. As anyone been quite out of the question for a do- receiver mechanism was coupled to a mi- who has.