THE FARNSWORTH MULTIPACTOR TUBE
                  Farnsworth's Cold-Cathode Electron Multiplier Tube
                            Uses Neither Grid Nor Filament
                                By Arthur H. Halloran
                               From RADIO October, 1934


Think of a vacuum tube without filament or grid, thus requiring neither an A nor
a C battery, which generates high-frequency oscillations!  This is what Philo Taylor
Farnsworth II, the television genius, accomplishes with the cold-cathode tube which
he originally developed as a current-amplifier for use with his cathode-ray pickup
tube.  It is also an exceedingly efficient detector and modulator.
     Its first public use as an oscillator was in a radio circuit whereby
communication was maintained between San Fransisco and Honolulu and between San
Fransisco and New York on September 13, 1934, over the Globe Wireless 35-meter
channel.  In this test, with 30 milliamperes at 1100 volts on the anode, the tube
drove a pair of 150-watt tubes in the final amplifier of a transmitter at the Heintz
& Kaufmann factory in South San Fransisco.  The signals were received at both
Honolulu and New York, and were reported as R9 by a ship 500 miles west of Honolulu.
Wilkens of Dunedin, New Zealand, also heard the transmissions.
     Previous laboratory tests proved that the tube is capable of generating
oscillations of any desired frequency throughout the range from 200 kilocycles to 60
megacycles, these limits being set only by the dimensions of the available tuned
circuits.  An undistorted output of 25 watts was obtained from an input of 25 watts.

AS AN AMPLIFIER
     The performance of this new type of tube depends upon the emission of secondary
electrons from two cathodes which are bombarded with high-velocity primary electrons.
The cathodes are coated with caesium silver oxide to enhance secondary emission.
They, together with a central ring anode, are assembled in an evacuated glass tube.
The tube is placed within a solenoid which is supplied with direct current so as to
maintain an intense magnetic field throughout the length of the tube.  When used as
an amplifier, a high frequency voltage is applied to the cathode terminals and a D.C.
voltage is applied to the anode terminal to hold it at a positive potential with
respect to the cathodes, which are shunted by a coil and variable condenser in
parallel.  The shunt circuit is tuned so as to be in resonance for the applied high
frequency voltage.
     When the D.C. voltage is applied to the anode terminal, any free electrons in
the inter-electrode space would immediately be drawn to the anode were it not that
the longitudinal magnetic field neutralized the transverse electrostatic field from
the anode and were it not for the high-frequency electrostatic field which draws them
to the alternately positively charged cathodes.  The strength of these several fields
can be adjusted to allow an electron to be shuttled back and forth in a zig-zag path
between the cathodes any desired number of times before it is finally drawn out of
circulation at the anode.
     Each time that a high velocity electron strikes a cathode it causes the emission
of from 2 to 8 secondary electrons, the number of secondaries depending upon the
velocity of the impacting electron, and thus upon the amplitude of the voltage
applied to the cathodes.  Each emitted secondary likewise causes the emission of more
secondaries, so that the process is rapidly cumulative and gives rise to a tremendous
amplification of current.
     In the foregoing simplified account of the tubes operation as an amplifier, one
important factor has been omitted.  The anode attraction, which causes an electron to
leave the vicinity of a cathode and which accelerates its velocity as it approaches
the plane of the anode, also decelerates its velocity as it leaves the anode plane
and approaches the second cathode, which is now positively charged so as to attract
it.  Its resultant velocity may therefore not be sufficient to cause emission from
the second cathode.  To insure emission, additional energy must be imparted to it,
this energy being obtained from that stored in the resonant circuit, as indicated in
the accompanying circuit diagram.
     The high-frequency supply is of the order of 50 megacycles and is loosely
coupled to the tuned circuit so as to apply from 25 to 90 volts across the cathode
terminals.  The positive potential on the anode may be 100 volts or more, depending
upon the desired current output.
     The tube has a discontinuous voltage-current output characteristic with a series
of successively higher current peaks as the voltage is increased.  Maximum current
output is obtained when the anode voltage is just sufficient to allow an electron to
travel from one cathode to the other during 1/2 cycle of the high frequency
excitation.  Other successively lower peaks occur at anode voltages corresponding to
transit times of 3/2, 5/2, 7/2, 9/2 cycles, the last being the least which has yet
been measured.
     The external magnetic field is unnecessary when the cathodes are properly curved
instead of being plane.  Their curvature can be calculated to focus the electrons
automatically for specified anode and cathode voltages.  This eliminates the need of
a D.C. supply for magnetic focusing.
     The tube's theoretical output is twice that of an equivalent hot-cathode tube
operated as a Class A amplifier.  Its practical output is limited by the ability of
the cathodes to withstand the high temperature to which they are subjected by
bombardment from a rapidly increasing number of electrons.  One test of a small tube
showed an output of 100 watts of undistorted energy before the cathodes were
destroyed by heat.  Such destruction is prevented by means of resistors in the
cathode leads.  A small tube can be safely operated so as to deliver 45 milliamperes
with 200 volts on the anode.  It is to be noted that the tube operates as a current
amplifier and that the amount of voltage amplification is dependent upon the
resistance in the output circuit.
     The theory of the tube's operation as a detector or modulator should be evident
from the non-linear voltage-current characteristic and requires no elaboration here.
It is especially sensitive in the detection of ultra-high frequencies.  [Editor's
Note: P.T. Farnsworth III told Eric Dollard that the multipactor tube and the Tesla
coil were a marriage made in heaven! TB]
     It is a well known fact that any amplifier circuit generates oscillations when
arranged to furnish an input voltage of proper magnitude and phase.  Consideration of
the manner in which Farnsworth's tube functions as an amplifier shows that it
conforms to this requirement when connected to a resonant circuit which is tuned to a
frequency whose half-period is equal to an electron's time of transit, as determined
by the frequency of the oscillations applied to the cathodes.
     But the great value of the Farnsworth tube resides in the fact that it is self-
exciting, i.e., that it requires no external high frequency voltage when used as an
oscillator.  Aside from the energy which is required for the magnetic focusing field
and which may not be needed eventually, the only external source of energy is that
which maintains a positive potential on the anode.  It apparently represents a new
discovery in vacuum tube phenomena.  Among engineers there is a difference of opinion
as to its cause.
     One plausible explanation is based on the assumption that there is no
appreciable space charge effect in the tube when oscillations start.  There are
always some free electrons present in the inter-electrode space, if only those due to
photoelectric emission from the cathodes.  These are attracted by the anode when it
becomes strongly positive but are prevented from immediately going to it by the
longitudinal magnetic field.  Their acceleration as they approach the plane of the
anode causes a current to flow, through half the inductance coil in the tuned
circuit, to one cathode.  This provides an out-of-phase voltage drop which
accelerates the electrons toward the other cathode with sufficient velocity to cause
secondary emission therefrom.  The emitted electrons then establish a current flow
through the other half of the mid-tapped coil and cause a voltage drop in opposite
phase so as to accelerate the electrons toward the first cathode which is thus caused
to emit more secondaries.  Repetition of this process quickly builds the current up
to a point where it can be delivered to the output circuit without stopping the
internal oscillations.  The oscillating frequency is that to which the resonant
circuit is tuned.  This explanation has not been confirmed by physicists, but is
presented only as a means for visualizing possible actions in the tube.  When the
engineers disagree, the physicist must experiment.
     Much work has yet to be done before standardized tubes will be available for
experimental use.  Television Laboratories Ltd. has licensed two factories for
commercial production.  But it will probably be a matter of some months before tubes
are available for amateur use.

                                        finis


     Mrs. Elma "Pem" Farnsworth has recently lent Eric Dollard and Borderland a
Multipactor Tube for testing in our lab.  The Multipactor holds promise in the "free
energy" field as a truly workable electronic device that may produce over 100%
efficiency.  The test as noted in the above article claims that "an undistorted
output of 25 watts was obtained from an input of 25 watts."  This does not take into
account radiated heat and possible residual losses.  The Multipactor that Borderland
is testing is a perfected model of the unit referred to in this article.  It appears
as though it actually was used in radio frequency broadcast equipment.  Preliminary
tests show the tube to be good and further tests are being planned.  More information
will be released through this Journal as we progress on this project.

     For those interested in obtaining more information on the Multipactor Tube the
related U.S. Patent numbers for P.T. Farnsworth II are: #1,969,399 - Electron
Multiplier, Aug. 7, 1934;  #2,071,515 - Electron Multiplying Device, Feb. 23, 1937;
#2,071,517 - Multipactor Phase Control, Feb. 23, 1937;  #2,135,615 - Multipactor,
Nov. 8, 1938;  #2,140,285 - Multiplier Coupling System, Dec.  13, 1938;  #2,140,832 -
Means and Method of Controlling Electron Multipliers, Dec. 20, 1938;  #2,141,837 -
Multistage Multipactor, Dec. 27, 1938;  #2,141,838 - Split Cathode Multiplier Tube,
Dec. 27, 1938;  #2,143,262 - Means for Electron Multiplication, Jan. 10, 1939;
#2,161,620 - Two Stage Electron Multiplier, June 6, 1939;  #2,172,152 - Radio
Frequency Multipactor Amplifier, Sept. 5, 1939;  #2,174,487 - Self-Energized
Alternating Current Multiplier, Sept. 26, 1939;  #2,179,996 - Electron Multiplier,
Nov. 14, 1939;  #2,203,048 - Shielded Anode Electron Multiplier, June 4, 1940;
#2,204,479 - Means and Method for Producing Electron Multiplication, June 11, 1940;
#2,217,860 - Split Cathode Multiplier, Oct. 15, 1940;  #2,260,613 - Electron
Multiplier, Oct. 28, 1941;  #2,274,194 - Apparatus For and Method of Electron
Discharge Control, Feb. 24, 1942; #2,286,076 - Electron Control Device, June 9, 1942;
#2,311,981 - Electron Control Device, Feb. 23, 1943.
     The work of Philo Farnsworth II is as important to energy researchers as that of
Nikola Tesla.  All of us at Borderland wish to thank the Farnsworth family for
allowing Eric Dollard access to the technology.  There is much yet to be learned from
the work of P.T. Farnsworth II.  Among his outstanding achievements he developed
television and also a way to produce nuclear fusion electronically (see U.S. Patent
#3,386,883 - June 4, 1968).

     Pem Farnsworth has joined with son Kent in the Pemberley Kent Publishers, Inc.
The current project is Pem's biography of her husband Philo II (whom she calls Phil)
which is also an autobiography of her life with him.  From the chapter outlines that
I have seen so far this promises to be an excellent book which is not just the story
of the invention of television but a fascinating view into the life of an original
thinker and inventor and the struggles in his life to manifest his ideas against
great odds.

     Advanced sales of the Collectors Classic Edition (a limited, numbered edition of
500) of the as yet untitled biography, along with the sale of stock in Pemberly Kent
Publishers, Inc. are now taking place and will be the means by which the regular
edition of this book will be printed.  From the Pemberly Kent press release:

     "As the publicity we have planned and anticipate will surround the placement of
the statue of Philo T. Farnsworth II in the Capitol in Washington, D.C. begins to
grow, we feel the Collectors Edition is highly appropriate and will sell out
completely in advance of the publication date (3-10-89).
     "The Collector's Classic Edition will be impressive.  There will be a quality
cover with embossed gold lettering.  Each edition will be accompanied by one of the
last 500 Philo Taylor Farnsworth II commemorative stamp covers cancelled on the First
Day of Issue in the office of the Postmaster General in Washington, D.C.  The edition
will include a fine portrait of Philo and be personalized and autographed by the
author, naturally.  Pem is working very hard to bring a very special story into this
world, she has a powerful and moving story here.
     "Friends and relative were, of course, given first opportunity to purchase this
special edition and they have responded dramatically.  The promotion of the
collector's Edition in Idaho on the 13th & 14th of May, 1988 will be the last limited
offering.  The public offering to collectors, speculators, researchers, etc. will
commence immediately after the festivities in Idaho.
     "Despite the unexpected demand for this edition, we intend to stand fast in our
promise to limit this edition to 500 numbered reproductions, in deference to the
serious collector or speculator.  Who knows what this edition will be worth after the
Philo's statue is unveiled in Washington, D.C.?
     "As each issue is paid for, a certificate of authenticity will be issued stating
to whom it will be personalized and the specific edition number he or she will
receive.  If you are buying one or more for speculative purposes, state this when you
order and the personalization will be omitted.
     "The collectors edition currently has a $100 price tag.  There is a payment plan
available (four monthly payments of $25) which many have found very helpful.  There
is a likelihood of the price doubling after October 15, so your quick response on
this is appreciated."
     For the latest edition of the Pemberly Kent Newsletter send $2 to cover printing
and postage.  Anyone interested in more information regarding the Collector's Classic
Edition should, with all due haste and speed, contact:

               Kent M. Farnsworth, President, PEMBERLY KENT PUBLISHERS
                    2121 Garfield Avenue, Salt Lake City, UT 84108
                                Phone: (801) 583-9921^Z