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Matthias, F. T. (ed.) / The Wisconsin engineer
Volume 33, Number VIII (May 1929)

Morack, Marvin M.
The sharp eyes of electricity,   pp. 280-281

Page 281

first silvered and then coated with one of the alkali ma-
terials. Contact with the cathode is made by a flush seal.
  It has been found that the introduction of a small amount
of an inert gas, gives the tube an internal amplification
factor, i. e., an amplicication of the current. When a gas
is present in the tube some of the photo-electrons traveling
between the cathode and the anode collide with the gas
molecules and cause ionization by collision.  That is to
say that the molecule will be broken up into a positive ion
and a number of electrons. The free electrons will con-
tinue toward the anode with the original electron, while
the positive ion will be attracted by the cathode. Now,
in place of an electron traveling to the anode, we have a
number of electrons, thus increasing the current.  There
might be two or three or even more depending on the kind
of gas, the pressure, the construction of the tube, as well
as the voltage applied to the electrodes.
   When quantitive measurements are being made with
the photo-tube, the voltage applied to the anode is of
much importance.    Of course the number of electrons
emitted from the cathode is dependent on the incident
light, but the number that reach the anode is a function
of the anode voltage. If the voltage is low, only a few
electrons will reach the anode and the photo-current will
be small. As the voltage is increased, more and more
electrons will reach the anode until finally all the electrons
emitted will reach the anode. When this condition of
saturation is reached further increment in the anode voltage
will not increase the photo-current.
   If a gas is introduced into the tube the result changes.
 When the anode voltage exceeds that necessary to ionize
 the gas, more curent results. As the voltage is still further
 increased, more ions are produced and the current still
 increases. Finally, a voltage is reached where glow dis-
 charge results and the photo-current is independent of
                                      the. incide-nt lioht. Ob-
viously, t h e greatest
sensitivity of the tube
occurs just below the
glow voltage.
  T h e photo-electric
t u b e finds application
in industry because of
the fact that the cur-
rent which passes
through it is directly
proportional t o t h e
light flux actuating it,
and can therefore be
used as a m e a n s of
measuring the quantity
II.I .  -rE _ I A I TY
  FIG. 1: Photoelectric Relay Unit.  ot light. I is tact, we
                                     say, is connected with
the quantum theory of light in which the quantum is the
smallest possible unit of energy. One electron is emitted
for each quantum absorbed. Some modification is neces-
sary in the statement that the photo-current is directly
proportional to the incident light, since photo-sensitive sub-
stances are not equally sensitive to all colors of light. The
tube has maximum response for the blue portion of the
spectrum, though the maximum sensitivity falls in different
parts of the spectrum for different materials.
  The relative sensitivity of the photo-electric tube is
similar to that of the human eye. The average eye seems
to be most sensitive to a yellow-green color to a wave'
length of 555OAngstromes.    Of all the alkali metals used
as photo-electric tube cathodes, caesium most nearly ap-
proaches the sensitivity of the human eye.
FIG. 2: Photo-electric Apparatus for Grading Coffee.
  Let us consider what photo-electric tubes are doing in
industry. We find them in the factory, we find them in
the theatres, we find them on highways, in tunnels,-indeed
we may soon find them doing the tedious task of sorting
beans in South America.
  Of the numerous counting devices using photo-electric
tubes, in which the tube acts simply as an "on and off"
relay, the vehicle counter is perhaps the most simple. The
lamp, with its associated day light shield shown in Fig. 1,
is mounted at a convenient height on one side of the
highway. Directly across the roadway, and at the same
height is another box containing a photo-electric tube, an
amplifier, and a counter.
  The beam of light projected across the road causes the
photo-electric tube to pass current which in turn holds
the grids of the amplifier tubes biased negatively so that
no current flows in the relay    coil.  When    a vehicle
traveling in either direction along the highway intercepts
the beam of light, (Fig. 1) the photo-electric tube is shut
off and the amplifier is full on. The plunger is thus
pulled down and the counter registers the passing of the
vehicle.  As soon as the vehicle has passed, the bias is
again placed on the amplifier and the stage is set for
another count.
   The apparatus described above is very simple in oper-
 ation but its application need not be confined to counting
 vehicles. In our modern industry, boxes, apparatus, or
 numerous other articles, traveling on belt conveyors, may
 be counted in the same manner.
   In order to show how the development of the photo-
 electric tube centers upon our every-day life, let us select
                   (Continued on page 308)
MAY, 1929

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