September 1932 Radio News
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"We might say almost, that all modern scientific investigation in
the basic sciences, and a good deal of all practical developments,
are connected in some way or other with the electron." So writes
Dr. Irving J. Sax in 1932 regarding the incentive for determining
as precisely as possible the mass and electrical charge of an electron.
Bohr atomic model had just been introduced two decades earlier,
so the entire concept of particle physics was still in its infancy.
As with most areas of physics, experiments are conducted with particular
biases and expectations such that often the results are manipulated
as needed to conform to preconceptions. Look no further than the
complex retrograde motion planetary models devised and perfected
by early astronomers who believed the universe revolved around the
Earth. It wasn't until a heliocentric model was accepted (following
the exile and execution of many 'heretics') that orbital systems
became simplified and readily explainable. The same, to some extent,
has been true of subatomic particle physics, except digging deeper
into the true nature of the beast has continually revealed greater
and greater complexity. Each time a new 'fundamental' particle is
discovered through empirical testing, an new, more basic element
is theorized and must be explored; to wit, the Higgs Boson (aka
'God' particle). Indeed, even the planetary-like Bohr model of the
atom was proven incorrect by the advent of quantum mechanics and
the ensuing electron orbital cloud model based on
probability distributions (in 1932, the year of this article
in fact, by Mullikan). It was a mere five years prior to the publishing
of this article that Heisenberg had formulated his 'Uncertainty
Principle' that limited the precision with which a particle's
speed and mass could be determined. None of this subtracts from
the importance of each step along the way, and in fact, augments
the need to continue investigating the fundamental aspects of nature's
building blocks. For the particle physics historian, this article
will prove to be a treasure trove of information.
See all articles from
An Interesting Experiment in Weighing the Electron and What
It May Mean
By Dr. Irving J. Saxl
He Made the Experiment
Figure 1. - Dr. Felix Ehrenhaft, Professor of Physics
at the University of Vienna
In all our modern electronic technique, including radio technique,
television, acoustics and in all other basic sciences connected
with electricity we are making use and apply constantly the electronic
theory and its fundamental unit, the electron.
We have considered the electron to be the smallest entity of
matter, having one single, definite electric charge and include
it in our formula, whether we are now building radio tubes, X-ray
tubes, motor-generators or whether we are making intricate electrical
measurements. We might say almost, that all modern scientific investigation
in the basic sciences, and a good deal of all practical developments,
are connected in some way or other with the electron.
We are living in a century of electrification. New electrical
machines, appliances and instruments are still being given too humanity
at high speed - and all of them go back, in their last root, to
our knowledge of the electrical phenomena and its basis: the electron.
But what do we actually know about this tiniest entity of the
universe? It is the commonest thing in the cosmos, there is no atom
of matter in which there is not at least one electron, and yet,
what is it in reality, this most important, minutest quantity of
The theory that a material body is composed of tiny "building-stones,"
which cannot be divided, is not new. The Greek philosopher, Democritos,
had written of it already and even in the Hindu philosophy, first
signs of this idea appear in a general way.
Today there are a number of units which we cannot subdivide chemically.
We call them elements. From Faradays experiments on electrolysis
it was found what amounts of a certain material can be electrolytically
deposited within a certain time. It is now possible to determine
the number of molecules of which the deposited substances are composed
by using the laws which have been given us by Loschmidt, making
it possible to determine, mathematically, the number of the molecules
contained in each cubic centimeter. Using this number, called "Loschmidt's
Number," and using the data taken from Faradays' laws it has been
possible to determine the average charge of a single ion. It was
found to be in the order of 10-10 electrostatic units.
Around the year 1900, Townsend, J. J. Thomson and A. H. Wilson
carried on further determinations of average values which also gave
an electric charge of about 10-10 for the monovalent
Special Condenser and Vaporizer Employed
Figure 2. - Left, shows a cross-sectional diagram of
the condenser in which the movement of the particles was
analyzed. Figure 5, right, contains a cross-sectional sketch
of the vaporizer employed for producing the particles to
be "shot" into the condenser
Of course, as far as electronics technique is concerned, the
single electron does not seem to be today of industrial importance.
What is important is the effect of a vast number of electrically-charged
bodies which, after all, make important for practical use only the
average value. For going deeper into the matter, however, it is
important to know more about the individual happenings within a
physically defined body.
If we have a carload of potatoes, can we say the size of the
potato is this and that? Is it not probable that, if we observe
a large-enough number we will find small ones and large ones ...
and that the small ones might be almost any size smaller than the
larger ones? Can we take the "average potato" and say: this is THE
size of the potato?
Can we, therefore, state positively that there is no smaller
charge in the world? We have subdivided the molecules and we have
subdivided the atoms which, as the name expresses, "cannot be subdivided
any further." Is the electron, on the knowledge of which we base
so much of our present-day physical knowledge, is it really the
last bit of matter? Or is it just one step farther into the unknown
depths of the universe?
It was Dr. Felix Ehrenhaft, Professor of Physics at the University
of Vienna who wrote in 1909: "Smallest entities of electricity are,
as far as can be predicted, to be expected upon particles of smallest
capacity" and, "These particles, however, have to be large enough
to make them just individually perceptible optically, as it is necessary
to investigate each one separately." *
For determining the size, the weight of these particles, the
relation between their electrical charge and constituting matter,
the important entity e/m, , we naturally cannot apply a chemists
balance. Even the finest balances for Pregls micro-analysis are
crude in comparison with the values which have to be determined
for this purpose. Other ways have to be found for bringing about
However, a condenser balance has been developed making possible
the incredible exact weighing of body-particles smaller than the
tiniest dust particles which we see dancing in an intensive ray
of sunlight if we look normally upon this projecting ray.
The test particles which are to be investigated are brought into
the field of an electric condenser. They are strongly illuminated
from the sides and are viewed through a microscope. By putting electric
charges across the condenser plates it is then possible to move
these particles up and down between the plates by electrostatic
attraction and repulsion.
From these movements of the particles in the condenser, the exact
time of which is determined, it is possible to calculate both the
weight and the charge of the body as described in greater detail
at the end of this article.
Acting upon his idea, Ehrenhaft tried to take into consideration
for his experiments, particles of well-defined form and chemical
characteristics. He therefore worked with small globes of gold,
platinum, silver, mercury, etc. The spherical form of his particles
he was able to show by microphotographs in white light. For fully
resolving still smaller bodies, the author has helped perfect for
him the application of ultra-violet-microphotography, following
early designs of Prof. A. Koehler.† A microscope using these
principles has been manufactured by Zeiss and used in this country
for the optical analysis of alloys.
The Set-Up for Weighing the Electron
Figure - 3. This is a front view of the observer's table
in the Ehrenhaft experiment. Observation takes place through
the microscope, Number 7, the illumination of the tests
particles being normally made at right angles by the powerful
projectors, Nos. 1 and 2.
For making visible these very small particles, Ehrenhaft constructed
a special tiny condenser. It consists principally of two round plates
of brass or iron of about 1/4 inch in diameter and spaced about
1 millimeter apart. These condenser plates form the walls of a very
small air chamber (see Figure 2 which shows a schematical cross-section
through the condenser that was used in all the tests on the particles).
The two cylindrical pieces, D1 and D2,
of which the condenser is made, are screwed in from the upper and
from the lower ends. By means of the screws, S1 and S2,
it is possible to correct their distance precisely. 0 is the front
lense of the microscopic objective through which the particle is
viewed. The illumination takes place, normally, upon the cross-section
of the condenser and upon the axis of the observing microscope,
as shown in the close-up of the front of Ehrenhaft's apparatus in
Figure 3. The observation is made between the condenser plates D
which are embodied in a housing of Bakelite. T1 and T2
are the terminals into which the contacts of the electric conductors
are screwed securely. The gas, which is chemically and physically
purified and in which the test particles are suspended, is brought
into the viewing condenser by means of the stopcock A.
The Set-up Employed
Figure 3 shows the front view of the apparatus at the eyepiece
of which one observer is to sit. 1 and 2 are the illuminating arcs.
These are high-intensity, self-regulating, direct-current arc lights
burning with about 30 amperes. It is necessary to use 2 separate
sources of illumination as during the long duration of the observations
one pair of carbons may burn out. A second pair of carbons is therefore
always ready for use in the other projector so that it can be put
into use immediately and make possible a continuous observation.
In addition, these two light sources put against each other in an
angle of 180 degrees are necessary also for another reason:
Most particles brought into the path of this highly intensive
light react in a specific way upon the irradiation. Dependent whether
they are light-positive or light-negative they move to the light
source or run away from the source of light. (This effect, called
Photo-phoresis and probably in definite relation to the photo-electric
principles involved in photocells, was discovered by Ehrenhaft.)
By using two separate lighting units, diametrically opposed to each
other, it is possible, simply by illuminating from the opposite
direction, to push a particle back into the center of the observation
field. This change in illumination is done with the aid of the electro-magnetic
shutters, 3 and 4, which are operated from a double-pole switch,
The particles, which have been formed in the desiccator, 6, are
brought through glass tubing into the condenser. The microscope,
7, is the device through which the particles in the condenser are
observed. The actual illumination takes place from the left and
the right. For eliminating the effect of infrared heat rays, the
light, after coming from the projectors and the shutters, passes
through two filters containing a solution in the horizontal cylinders
8 and 9. The light beam is concentrated into the condenser field
with the aid of two microscopic objectives, 21 and 22, so that an
extremely intensive "dark-field" illumination is secured. For avoiding
any indirect heat-radiation, the two projectors are placed behind
the asbestos walls 10 and 11.
The Assistant's Recording Table
Figure 4. - In these intricate investigations the incorporation
of an assistant for recording is imperative. The assistant
records time upon the accurate electrically-operated stop
watches, notes the applied voltages, gas pressures and makes
other important records
Time Recording Important
The amount of voltage applied across the condenser plates can
be regulated by the rheostats 12 and 13. The switch, 14, makes it
possible to change, instantaneously, the polarity between the two
plates. With a foot switch, 17, this voltage is put onto the condenser
The telegraph keys, 15 and 16, at the left lower side of the
picture, operate automatic stop-watches electromagnetically. These
watches can be read down to one-fiftieth of a second. They record
the time of ascent and descent of a particle within the condenser,
the path of which is viewed upon a grid in the observing microscope
7. This path is not a straight line, up and down, but moving in
different curves. The particle dances about following the Brownian
movement; a twinkling spot upon a dark background in the observing
The exhaust gauges, 18, consist of a series of small capillary
tubes through which the exhaust gas has to pass and by means of
which it is possible to regulate the speed of the exhaust procedure.
On the manometer, 19, the air pressure in the electric condenser
can be read (through a little telescope which is on the assistants
desk). 20 is a McLeod, an instrument with which pressure (of a fraction
of one millimeter) can be read down to microns.
The observer looks into the eyepiece of the microscope so as
not to loose the path of his particle. He is kept very busily engaged
in regulating the voltages which change sometimes, especially if
an accidental radioactive material has been in the neighborhood
of the condenser or other effects have taken place as to liberate
electric charges, for instance, by irradiation with ultraviolet
light. The observer has furthermore to start and stop the procedure,
to regulate the exhaust, to work the light switches so that he cannot
afford the time to make actual recordings. Mechanical recording
has therefore been applied for these intricate investigations, under
the control of an assistant.
Figure 4 shows the assistant's table. It is placed behind the
observer's seat, as at the assistant's table lights are necessary.
Meanwhile the entire room is kept dark so that the observations
will not be effected by any outside light. In Figure 4, 1 and 2
are the electro-magnetically-controlled stop-watches.
The actual voltage put across the condenser plates and regulated
by a shunt, is read on the precision voltmeter 3. The air pressure
in the condenser is read over a system of reflecting mirrors with
the small telescope 4.
Producing the Particles
The test particles were produced by three different methods:
No. 1. The first method was to build an electric arc between two
pieces of metal. This arc smelts off minute drops of the metal to
be used as particles. They evaporate from the electrodes and coagulate
in the colder atmosphere which surrounds the electrodes in the form
of balls of the dimension 10-4 to 10-5 cm.
No.2. The second method employs bodies which can be evaporated and
these are brought into a small container of glass or quartz and
heated with a small flame (Mercury, Sulphur, Selenium, etc.) to
liberate a steam which condenses in the form of small balls. No.
3. Figure 5 shows the third method of producing balls of a heavy
fluid as e.g. Barium-Mercury-Iodine, Mercury, etc. These small balls
will always be geometrical spheres of great accuracy where the capillary
powers are larger than the forces which bring about the "drop" form.
A vaporizer V in which specially purified gases are sucked through
the opening with the aid of a syringe-like pump, S, distributes
small particles of the fluid in which it is immersed.
How Gas Pressure Affects Fall
Figure 6. - Diagram showing graphically the dependency
of the speed of fall of particles from the average three-mean-path
of the gas as produced at different pressures from experimental
data of Dr. Max Reiss
The specific weight of this fluid is known and therefore also
the specific weight of the particles. These fall very slowly, due
to their sub-microscopic size. They are contained in the gas current
and are transported together, with it, through the funnel, B, into
the condenser field.
There the particle is irradiated strongly from the side, as stated
above, and it is seen in the microscope as a luminant point upon
a dark field. It is possible to read the distance through which
the particle falls freely in the air-condenser chamber, upon a grating
which is inserted into the eyepiece. If the particle carries an
electrical charge and if the plates of the condenser are charged
electrically, it is possible to move the particle upward by electrostatic
power! It is also possible to measure here by its speed!
The particle is allowed to fall again after switching off the
electrical field so that it moves downward under the influence of
gravitation. This procedure is repeated long enough so as to receive
satisfactory averages of the ascent and descent of each particle
If a sphere falls in a space filled with gas, its speed of fall
does not get larger into the infinite. The friction of the sphere
in the air works against gravitation and after a certain time, the
friction gets so large that the ball moves with a constant speed.
With a submicroscopic sphere as used in Ehrenhaft's experiments,
this constancy is reached after a very short time.
Investigations have shown that the velocity of such a particle
is proportionate, within certain limits, to the power acting upon
it. Stating v for velocity and P for power, we get the equation:
(1) v = P.B.
where B is a factor of proportion. Its physical meaning is the
velocity under the influence of the power which is unity. B may
be called the mobility of the particle.
For a free-falling body of spherical form we have, therefore,
where Vf is the velocity of the fall, a is the radius, σ
is the density and g is the gravitational acceleration.
If the particle is pulled upward again under the influence of
the electrical forces put unto the condenser and has the velocity
vs, then we can say:
where e is the electrical charge of the particle and E is the
field intensity in the small condenser.
From equation (2) can be computed the radius of the particle:
It is necessary to know, from other sources, the value of B,
of the mobility of the particle. This known, it is possible to determine
the electric charge of the particle, by simply inserting the value
for a, from the equation (4), and the value for E (which we can
read directly on the voltmeter) in the equation (3).
The mobility B has, therefore, to be found in some other way
if we want to determine the radius and the electric charge of our
particle. Stokes, the English mathematician, calculated for the
resistance W - which is reciprocal to the value of the mobility
B - which is impressed upon a sphere during its motion through a
It is assumed hereby that the fluid sticks continuously to the
surface of the sphere.
For a sphere, however, which moves in a gas instead of a fluid
and with reasonable speed, too, it may not be correct to suppose
that the medium will stick to the surface. Aeromechanics and hydro
mechanics have shown that this case practically never happens. The
so-called "laminar"-movement takes place only under theoretical
conditions, assuming an almost infinitely small movement of an inelastic
body in a fluid of small hydraulic mobility. Practically in all
cases eddies and whirls appear which disturb markedly Stokes' law.
Experience teaches against the theory that the gas glides along
the surface of the exposed body and, in addition, the formation
of whirls takes place for gases which are so dense that the average
free-mean-path of their molecules is small against the radius of
the sphere (that means sufficiently smaller than 10-5
It has been necessary, therefore, to correct Stokes' law. Following
the calculations of Stokes-Cunningham, this formula reads:
Where l is the average free-mean-path of the gaseous molecules
and A is a constant (which gives a value for the gliding and is
supposed, following the theoretical calculations, to be near unity).
Will experiments at different gas pressures especially in compressed
gases, be able to solve this problem?
Investigations at different pressures under one atmosphere have
been already made. Figure 6 shows a diagram of measurements which
have been taken for each individual particle at different pressures
by Dr. Max Reiss. From these experiments which have been made between
1 atmosphere down to about 50 mm. pressure, it seems that some particles
have a smaller density than the molecular material. On the other
hand, particles have been found, which indicate much smaller values
for the supposedly standard electric charge of an electron! It has
apparently been possible to measure single electrical charges as
low as 1.10-10 electrostatic units, that is less than
one-fourth of the values found by other investigators.
For giving an idea about the incredible small forces with which
we have to deal and which we have to control experimentally in these
intricate investigations, it may be mentioned that the forces which
act upon the particle are of the dimensions of 10-10
dyne. This corresponds to an attraction with which two containers
of about one quart of water each act upon each other over a distance
of about 2 miles!
If the particles with which he operated were small enough, Ehrenhaft's
measurements on individual particles showed values for the separate
electronic charge which went far below the quantum charge which
is required by the theory. From other methods the value of the electronic
charge was determined to be about 4,77.10-10 electrostatic
units. Dr. Robert A. Millikan in Pasadena who described the condenser
method at about the same time as Ehrenhaft††, and
who was awarded the Nobel prize found this larger charge.
In addition to having found smaller charges than the elementary
quantum Ehrenhaft states that it is not directly possible to consider
the electric charges he found, as simple multiples of the elementary
charge. These conditions of being able to build simple multiples,
quanta, would be a necessary requirement, if one of the cornerstones
of modern physics should be a true natural law: the quantum theory
of Max Planck which has proven so valuable a tool for many investigations.
According to Ehrenhaft, it would be necessary to determine the
value of the unit of negative electricity much lower than 47.10-10
electrostatic units ... provided that there is existing any atom
of electricity at all.
In practice we continue, today to use the electron as such in
our calculations and our engineering. But will we, under these circumstances,
consider the electron as a truly existing standard entity ... in
the scientific world of tomorrow?
* Wiener Akadem. Anz. number 7 March fourth,
† Ehrenhaft & Wasser, Philosophical Magazine.
Vol. 11. 1926.
†† R. A. Millikan: Physical Review
XXIX, p. 260, December, 1909, F. Ehrenhaft: Anzeiger d. Wiener Akademie
d. Wissenschaften March fourth, 1909.
Posted April 15, 2014