WHY DO CRYSTALS RECTIFY?

Radio Record, Volume I, Issue 7, 2 September 1927, Page 15

WHY DO CRYSTALS RECTIFY?

A FEW ‘SHIEORIES. Crystals have now been used tor the rectification of wireless signals for about twenty years, but it is curious to note that, long befure being put to this use, the crystal had been carefully investigated by scientists as a oneway conductor of electrical impulses of high frequency. Despite these facts, the present-day crystal investigator is almost as much in the dark as to how rectification takes place as was his predecessor of re-wireless days. Since the introduction of the crystal as a radio rectifier, it has teen found to possess other remarkable characteristics which are in cvery sense quite as mysterious in their nature as its rectifying properties. Besides being able to rectify I1.F. cnrrents, there are a number of crystals which are now known to be able to generate oscillations when they are connected

in suitable circuits, and, tm adaition, several varieties of crystalline minerals have recently been shown to possess the extraordinary property — of converting light rays into electrical impulses. Other strange and = unusual characteristics which are — possessed by some varieties «f ervstals show themselves in the cilects of "pyro" and "piezo" electricity, — That is to say, crystals possessing these properties are able to generate electrical | currents in their interiors when they are either heated or subjected to varying amounts of pressure. Yet here, again, practically nothing is known about the causes of these effects. One can only surmise certain vrobable actions whieh might account fer — the effects Theories whieh have Leen advanred in connection with crystal rectifieation qnay be divided into four distinct classes, as follow :-- 1, Theories assigning the cause of | erystal rectification to abnormalities

| in the electrical conductivities of materials } 2. Electrochemical theories. } 3. Thermal theories. 4. Molecular theories. Probably the most popular and well-known theories at the present time are those which can be included in the first division. In 1907 Professor Pierce found that a carbornundum crystal rectifier appeared to disobey Ohim’s law entirely, For instance, with a local potential of two volts applied to it, the crystal passed approximately 35-40 times: as much current in the one direction as it did in the other; and, furthermore, it was subsequently discovered that, as the loc--ally-applied potential was ‘creased, the one-way conductivity of the crystal also increased quite out of all proportion. Thus, with a local potential of 30 volts, the carborundum crvsital passed 3000 times as much current in the one direction as it «id in the other. At the same period other scientists were making discoveries of other crystals with rectifying properties when used in contact with a snit- |} able metal.

One electro-chemical theory assumes {hat there is present in the surface films at the point of crystal contact a quantity of absurbed air or gas, and it is owing to the peculiarities in the eonductivitv of this surface faver that the rectifving effect of the ervsal ig set up. The best known thermal theory has it that when the oscillatory current passes across the crystal contact a certain amount of heat is produced, giving rise to further minute currents of a unidirectional nature, and it is these currents that flow from the erystal into the JIuadphones of — the set. None of these theories taakes the slichtest attempt to explain the practically yerv important fact that many ervstals decrease in sensitivity after continued use, Nor do the theories offer any explanation as to why one erystal may he entirely satisfactory in sensitive properties, whilst another vevstal of an identical composition and form may be utterly useless for

-use im a broadcast crystal receiver. By floating fine crystal dust on the surface of mercury, and applying a ° fine eat’s-whisker to a particle of the ‘dust, good rectification has been ob‘tained, proving that conditions necessary for rectification must be present in the surface layers of the crystal, and not within its mass. Ft appears likely that the true explanation of crystal rectification will be based upon the surface molecular theorv, although it has not yet explained lhow the rectification is produced. Nevertheless, the holders of the theory appear to be making praiseworthy attempts to sccount for well-known properties of the crystal, which the exponents of the older conception of crystal actién simost entirely ignored