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450 Radio News for November, 1928
Magneto - Striction
An Interesting Field for the Radio Experimenter;
to Build a Magneto -Striction Oscillator
By M. J. Cztttler
RADIO NEWS LaGoratories
ccERROMAGNETIC" substances
(such as iron, cobalt, nickel and alloys
into which those metals enter in
considerable proportion) possess
"magneto- strictioe" properties; which means
that they undergo slight mechanical alterations
of shape, and some of their physical
properties change when they are subjected
to the influence of a magnetic field. This
action of the magnetic field on such sub
stances is manifested in a series of effects
discovered at different epochs and known
under various names.
The oldest among these is the "Joule effect,"
discovered by Joule about 1847; this
is the variation of the length of a ferromagnetic
rod exposed to a magnetic field.
Let us consider an iron rod freely suspended
inside of a long solenoid; an electric current
of constant intensity flowing through a
solenoid creates a magnetic field which is
practically uniform within the solenoid over
n great part of its length; provided that the
ratio between the length and diameter is
sufficiently high. This is illustrated in Fig.
1. The field strength within the coil is proportional
to the intensity of the current, and
the variation of the field may be governed
through the regulation of the magnetizing
current.
AN ALTERNATING ACTION
Let us now examine what happens to the
iron rod if the field inside the solenoid is
varied from zero upwards. First, an elongation
will take place. This will continue
until the field's strength reaches a certain
value, after which any further increase of
the field will cause a contraction; the rod
then becomes shorter, will again reach its
FIG.1
The magnetic field within a solenoid is prac,
Orally uniform in distribution, as illustrated
herewith.
11r. Outlier is shown here testing the magneto- striction oscillator described
in the accompany article.
original value, and will then continue to contract
until a saturation point is reached.
Any further increase of the field strength
will have no more effect on the length of
the rod.
The behavior of other ferromagnetic substances
under the same conditions will .be
different. Nickel, for instance, continuously
decreases in its length; while cast cobalt, in
contrast to iron, first contracts and then expands,
reaches its original length and continues
to elongate until saturation occurs.
The relation between the strength of the
magnetic field and the variation of the
length is shown clearly in Fig. 2. (Both
figures are taken from an article on magneto
-striction by S. R. Williams, published
in the Bulletin of the National Research
Council, August, 1922.
Other ferromagnetic substances may respond
differently, but one thing is common
to them all; they vary in length (whether
positively or negatively) with a rising field
strength, and reach a point where saturation
occurs, after which a further increase
of the field has practically no effect on their
length.
An important remark is to be made here;
the Joule effect is dependent on the direction
of the field. As the extent of the variations
in the length of such rods is extremely
minute, their measurement is a matter of
great difficulty; the utmost care must be
taken to avoid temperature variations and
changes in other physical conditions, which
may conceal the real values.
Various ingenious arrangements have been
used for such measurements; Fig. 3 gives a
schematic layout of the method used by
Professor Williams. The method of operation
is self -explanatory; the expansion or
contraction of the rod under test is converted
into angular rotation of the mirror
by means of the lever.
CORRESPONDING PHENOMENA
To the Joule effect corresponds another
phenomenon which is its opposite; the "Villari
effect." The forcible lengthening of a
ferromagnetic rod located in a magnetic
field is accompanied by a variation in its
magnetization or its permeability. As we
have seen in the Joule effect, various substances
behave differently under the same
conditions; but there is a relation, for each
metal, between its Villari effect and its
Joule effect. An investigator (Nagaoka)
who followed Joule, has confirmed his supposition
that a magnetic field has an influence
on the volume of a ferromagnetic
substance.
Another very interesting phenomenon
which belongs to the same group is the
"Wiedemann effect" and its two inverse effects.
The Wiedemann effect is the twisting
of a rod under the influence of a combination
of two fields, one longitudinal and
one circular. Suppose a magnetic rod is
clamped at one end inside of a long solenoid;
then a current passing through the
solenoid produces a uniform longitudinal
field inside of it. If, at the same time,
another current flows through the rod, a
circular field will be created around it. If
one of these currents is kept constant while
the other is varied, or if both currents are
varied, a rotation of the further end will be
observed.
The two corresponding inverse effects are:
first, a circular field is created when a rod
which is located in a magnetic field is sud-
These curves show how various metals change
in length under the influence of a magnetic
field.