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.
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'
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
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
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 50º 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 38º 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
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
Appearance of Rochelle Salt Crystals Made By the Author,
Undesiccated; Desiccated and Finally Dressed, Ready for Mounting in
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
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
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
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
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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