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MANY new and surprising things have come to light during the war. While the guns thundered in Europe American scientists quietly went about their work, hatching out many new marvels. One of the most unique inventions at least from the Experimenter's standpoint is the wonderful Piezo-electric effect of Rochelle crystals. In these, a veritable new gold mine has been opened to all experimenters. What these crystals will all do fairly staggers one's imagination. Think of a simple salt crystal that will actually talk, that can be a telephone receiver, and a transmitter combined, that will give as much as 100 volts when squeezed with two fingers, that will "sing" when current is supplied to it! And the best part is every one of us can make his own crystals and perform these experiments without having to buy this 20th century electrical wonder for as yet it can't be bought, anyway! We consider ourselves fortunate to present our readers with a very comprehensive article on the new invention by its inventor, Mr. A. McL. Nicolson. Many new uses will be found for the invention and we invite our readers to send us their experience for the benefit of all. The Editors.



"Speaking Crystals"
By A. McL. Nicholson

ELECTRICITY is liberated in many apparently different ways. There are at least a dozen so-called "sources" of electricity which are mentioned in text hooks. It probably could be shown that in every case in which electricity is produced, the fundamental cause of the manifestation is the same.

We have merely different ways of operating on matter with the dissipation of some form of energy; and the electric charge or current is generated. This signifies the transformation of one form of energy into another, and, generally, the mechanism is reversible. For example, in the case of static electricity the influence machine may be used reversibly as a converter" of electricity or of mechanical energy that is to say as a generator or as a motor. When we hear of a "new" source of electricity we are curious to find out all about the mechanism of the source as to its reversibility, efficiency of energy conversion, applicability to the mechanical arts, etc.

"Piezo-Electricity", discovered by J. and P. Curie, is the name given, in 1881, to a crystal source of electricity. In this case electric charges of different signs are liberated on different surfaces of a crystal when mechanical stresses or vibrations are impressed on certain of its parts. The word piezo, is derived from the Greek piezein, signifying "to press"; hence "pressure' electricity.


Crystals Which Evolve Electricity.

Not every crystal is piezo-electrically active. Nature grows crystals, whether mineral or organic, in thirty-two differing classes of structure of these, just twenty classes offer the necessary condition for piezo-electric activity. And this condition is asymmetry of structure or of molecular arrangement. While a very large number of crystals have the electric property, only comparatively few exhibit the phenomenon to a sufficiently interesting degree to warrant mention of it at this time.

Amongst mineral crystals quartz and tourmaline are the best known having the asymmetric structure the molecules consisting of regular "bricks", are staggered into a skew symmetry or spiral structure.

In the case of the organic crystals, the molecules themselves are asymmetric, that is to say, they do not comprise "regular bricks".

They have, in fact, little skew symmetries of their own. Both the mineral and the organic crystal exhibit their respective asymmetries as right- or left-handed skew structures. This is revealed in their power to rotate the plane of polarization of polarized light in corresponding right or left directions.

We are, lust now, more interested in the organic crystal since this type. comprising, as it does, the asymmetric carbon molecule, has proved to be more susceptible of large piezo-electric effect. Examples of active organic crystals are sugar, camphor, tartaric acid, etc.


Rochelle Salt Crystal Very Active Electrically.

A crystal which is very active piezo- electrically is "dextro rotary" sodium potassium tartarate or Rochelle salt. Recently, the Research Laboratory of the American Telephone and Telegraph and the Western Electric Companies investigated the possibility of developing piezo- electric crystals. It was soon found that crystals of Rochelle salt would give good results if prepared in a special manner.

Briefly, increased efficiency is brought about by the following conditions

1. Selection of particular habit of growth.
2. Desiccation. (Preserving by exhausting the moisture.)
3. Development of the crystal into a composite polar structure.
4. Application of static compression.
5. Use of electric poles normal to each other.
6. Application of torque.


How the Crystals are "Grown".

Rochelle salt crystals are grown from nuclei or seeds" of definite form. These are obtained by selection from crystal croppings spontaneously grown in a super- saturated solution of the salt (the formula is Na K C4 H4 O6. 4H2 0). The solution should be made up of 8 parts Rochelle salts to 5.33 parts of water. The density of the solution at 50 C. is 1.33 and the nuclei from which large crystals are grown, are "planted" in the mother liquor when the temperature has dropped to 38 C. The crystal grows rapidly as the temperature of the liquor falls to that of its surroundings. The seeds are selected so that they are practically square shaped the seed, for this purpose, must lie with its principal or optic axis in a horizontal plane.



When a crystal is grown very rapidly as by the temperature gradient method described it develops a composite structure termed by mineralogists the "hour-glass". Fig. 1 illustrates the entire process graphically.



Appearance of Rochelle Salt Crystals Made By the Author, Undesiccated; Desiccated and Finally Dressed, Ready for Mounting in Clamp.


How the Crystal are Mounted

Important use is made of this structure and its development is fully encouraged. It is found that if the crystal, when grown to the desired size, from 30 to 200 grams, and when thouroly desiccated, or dried, develops new and stronger electric poles on its surface.



Above: Crystal as a Phonograph Transmitter. Below: As a Transmitter or Receiver.

The vertical walls, surrounding the principal axis, farm one pole while the two horizontal, or basal planes, together form the other pole. Bees waxed tin foils serve as electrodes when applied to the crystal. Since compression greatly improves the piezo-electric effect in these crystals an appliance shown in Fig. 3 called the "spring compressor" may be used. The appliance comprises a pair of aluminium plates connected together with powerful springs. Thumb-screws are provided so as to apply 20 to 40 pounds pressure to the crystal. The compressor forms one pole, preferably the "grounded" pole of the crystal. The other is called the "girdle" pole, because fine wires may be stranded and wrapped around the crystal at its equator, making proper connection with the tinfoil coating there.

The mechanically sensitive regions of the crystal are at the four corners of the "square" on the ends of the two basal planes. Care should therefore be taken to have the crystal bear on these corners in the spring compressors. This is readily accomplished by filing the crystal on its basal planes so as to render it slightly con- cave on both crystallographic poles.

An ordinary half round file is used, and the top and bottom faces filed in such a manner that the corners are elevated slightly making them higher than the rest of the face.

When compression is applied to the crystal an electric charge is imparted to its poles so that the girdle electrode becomes plus and the basal planes, together (or the spring compressors) become minus.

This electrification will leak away and when the compression is relaxed, the crystal poles will reverse the signs of the liberated charges. Great sensitiveness is also obtained if the crystal is subjected to torsion.

A charge of several micro-coulombs and potentials exceeding 100 volts may be obtained by twisting the crystal with the fingers.


The Crystals "Talk"

If, now, we reverse the process and apply electric potentials to the crystal poles, sounds will he emitted by the crystal due to its relative displacement. It will be found, if a small mirror be applied with wax to different parts of a crystal and its motions examined by projecting a beam of light reflected by the mirror to a screen, that the principal component of motion is one of twisting. The crystal thus "wriggles" under electric stress and will emit tones in consonance with the potentials applied.

Several interesting experiments have been performed utilizing one or other or both of the electrical and mechanical effects produced by the piezo-electric crystal. We may first use the crystal as a transmitter or detector of mechanical vibrations or sound. A very convenient application is to the phonograph. A needle may be inserted in a plate attached to one end of the crystal, fig. 2, so that, if held properly over a moving record the needle will transfer torsional movements to the crystal, and corresponding electrical currents will be generated.



The alternating potential generated by the crystal under these circumstances may be as large as 10 volts and the resulting current will be several micro amperes. In the simplest form of this experiment, electro magnetic receivers of high impedance may be used to detect speech and music from the phonograph. Since the impedance of the crystal at acoustic frequency is about 300,000 ohms, the impedance of the receivers used should be very high at least a few thousand ohms. The crystal itself can operate several hundred receivers in series and parallel. See Fig. 4.

Instead of using the phonograph to agitate the crystal transmitter, we can, by adding a diaphragm to the crystal, talk or sing against the diaphragm and thus excite the crystal to about the same degree that obtains with the phonograph record. Thus by singing against the diaphragm near a resonant frequency of the crystal housing, say at a frequency corresponding to "middle C" in music, or 256 cycles per second, we can generate on alternating current in the crystal of 20 micro amperes and an open-circuit potential of 15 volts. The former may be measured with a transformer, thermocouple and D. C. micro ampere meter, the latter with an electrostatic voltmeter.

The diaphragm used is rather novel. A strip of gold-beater's skin or even of paper (stiff bond) is wrapped around the spring compressors, holding the crystal, as in the first experiment, and metal bands tighten the strip on each of the two spring compressors.



We now have a cylindrical diaphragm surrounding but not touching the crystal yet conveying stresses to the crystal thin the spring compressors. The diaphragm, in order to be effective, must be corrugated as shown in Figs. 3 and 3A. This is done very readily by twisting the bands holding the diaphragm in opposing senses prior to its stretching and tightening. Usually a separate and removable appliance is used to perform these operations on the diaphragm.

Vibrations, due to sound waves, proceed from the cylindrical diaphragm thin the spring compressors, to the enclosed crystal. The crystal, when it is disturbed by the vibrations, converts these mechanical effects into corresponding electrical charges or currents, which may readily be detected in the receivers placed in the circuit.


Crystal Transforms Electricity into Speech.

But, as shown above, another interesting function of the crystal is that of a receiver of electrical oscillations. If we impress an alternating potential on the crystal poles and use the transmitter construction just described, then the crystal, itself vibrating under the electric stresses applied, will impart axial and torsional vibrations to the cylindrical diaphragm. Thus, corresponding acoustic effects will be produced which may be heard considerable distances away from the crystal receiver.

This experiment, using the crystal as a receiver, may be performed in different ways. Most simply, a carbon microphone may be applied as transmitter. This is shown in Fig. 5 with a local battery and high ratio transformer, which matches fairly well the low impedance of the local microphone circuit with that of the high impedance crystal. In this arrangement the crystal operates as a "loud speaking" telephone and may be heard several hundred feet away.



Another experiment is the use of crystals at both ends of a line. In order to increase the detecting and emitting effects of the piezo-electric crystal, the vacuum tube amplifier may be used. Fig. 6 shows a crystal transmitter, a crystal receiver, and an intervening two-stage vacuum tube repeater or relay. Speech and phonograph effects applied to the transmitter may be heard proceeding from the crystal receiver, with sufficient volume to fill a large auditorium. Under these conditions, if the crystal receiver is placed nearer than a few yards from the crystal transmitter, the receiver will sing to the transmitter. This well known phenomenon in ordinary telephony is known as "howling" and is due, of course, to the presence of local free energy, of battery or amplifier, which will maintain circulatory oscillations of the acoustic and electrical effects.



At present the efficiency of these crystals. as defined by the ratio of the output to the input, is not high. This is principally because of the difficulty of associating a perfect housing with the crystal. The housing, is, of course, necessary in order to translate vibrations to or from the crystal. The efficiency becomes greater only when the frequency of the applied vibrations is close to that of the natural frequency of the combined crystal and its housing. Hence the comparatively large values of the alternating currents and potentials generated by the crystal when it is operated near any of its resonant frequencies for, it may have several modes of vibration.

Some day it may be possible to pick tip a pebble from the beach, place it to the ear and listen to voices spoken to another pebble found on some other shore.


[As transmitters and loud-talkers, these crystals produce remarkable results and the amateur will be well repaid by his results, provided he follows the instructions given in this paper. As detectors for wireless telegraphy, however, they are practically valueless, if used in the regular way. Perhaps some amateur may some day hit upon a scheme which will produce the much looked-for perfect crystal. EDITOR.]




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