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. The
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
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 microcosmos?
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 ion.
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
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 this determination.
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, 5.
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
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 plates.
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 microscope.
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
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
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
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 tested.
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, the
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 fluid:
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 cm.).
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, 1909.
† Ehrenhaft & Wasser, Philosophical Magazine. Vol. 11.
†† 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