How Luminous Tubes Work
What happens inside a neon sign
Fundamentally, the luminous tube is a very simple device. It
consists of an evacuated glass tube fitted at each end with a metal
terminal called an electrode. Inside the tube is a small amount of
highly purified inert gas. Connected to the two electrodes is a
source of high-voltage electrical power. When the current is turned
on, the tube glows with a steady, piercing light.
A practical person can learn how to bend the glass, attach the
electrodes, pump out the air, fill the tube with neon or some other
gas, seal it, mount it, connect it to the power source, and install
the completed tube. All these things can be done without knowing how
or why the tube works.
But what happens if the newly installed tube goes dead in two
weeks, or if the fluorescent tubing starts darkening and visibility
decreases? A great deal of future trouble can be saved by knowing
some simple facts about what the glow discharge is and under what
conditions it will deliver satisfactory service.
Sparks and glow discharges
The familiar glow of the neon sign is a first cousin of man's
oldest electrical acquaintance, lightning. The red glow of neon and
the blue flash of lightning are both electrical discharges in gas. In
the neon sign, the gas is neon. In lightning, the gas is the air: a
mixture of oxygen, nitrogen, carbon dioxide, traces of other, rare,
gases, and water vapor. In the luminous tube, the pressure is very
low, in fact, a partial vacuum. Lightning occurs in normal
atmospheric pressure. But the neon glow and the lightning flash are
electrically very much alike. They both illustrate the fact that when
electricity passes through a gas, light is given off.
If lightning and neon glow are so much the same, what makes their
light so different? The differences are due to the factors described
above; that is, differences in pressure and in the kind of gas used.
The lightning spark is hot and blue-white in color. It consumes and
transforms a tremendous amount of energy. By using a particular gas
at a reduced pressure in a glass tube, it is possible to produce a
steady glow, with very little power and very little heat.
The combination of penetrating light, high economy and design
flexibility makes the luminous tube a highly effective source of
light for advertising, lighting, and decoration. The large industry
based on luminous tubes, as outlined in the previous chapter, has
resulted from the practical application of this low-pressure
electrical gas discharge.
What happens inside the tube
The gas inside the tube consists of particles called molecules.
Under ordinary conditions, these molecules are electrically neutral;
that is, they are neither positively nor negatively charged. To start
the gas discharge, these molecules must be broken up into electric
charges. When this happens, a current can flow through the gas, and
this current produces the desired light.
When a molecule loses or gains one or more electrons, it is no
longer electrically neutral and assumes a positive or negative
charge. This charged molecule is called an ion, and the process of
gaining or losing electrons is called ionization. In any gas, there
is always a very small percentage of molecules that are naturally
ionized at any given time.
This natural ionization results from such sources as cosmic rays
generated from the sun, and natural background radiation that exists
on earth. Radiation dislodges electrons from the outer orbits of
electrons surrounding the gas molecule's nucleus. If an electron is
removed, the remainder of the molecule becomes positively charged,
and the result is the division of this small percentage of the gas
into positive ions and negative electrons.
Because like electrical charges repel one another and unlike
charges attract each other, a free electron is strongly attracted to
any nearby positive ion. This attraction leads to the rapid merging
of the positive ions and negative electrons into one neutral
molecule. During this process, the remaining electrons within the
molecule rearrange themselves, which causes the molecule to give off
light. This is the light we want to produce, and its color depends
upon the kind of molecule (or gas).
Thus, to produce a gas discharge, electrons must be removed from
neutral molecules and recombined with positive ions to form other
neutral molecules. The practical way of producing this ionization is
by passing a current through the gas.
How the current produces the glow
When voltage is applied to the electrodes, one electrode becomes,
momentarily at least, positively charged, as shown in Figure
1. The electrode attracts any free negative electrons that may be
present in the gas. If the voltage is high enough, the electrons will
be attracted with tremendous force and will accelerate toward the
positive electrode reaching speeds of thousands of miles per second.
Figure 1: Action of electrons and positive ions in
Before one of these electrons can get very far, however, it
collides with a neutral gas molecule that lies in its path. It hits
this molecule with such force that one or more (usually more)
electrons are liberated. These electrons, once free, start off toward
the positive electrode. They soon collide with other neutral
molecules, liberating still more electrons, which go off to smash
still other molecules.
Shortly after voltage is applied to the tube, the whole body of
gas is in motion. Electrons are liberated from molecules, free
electrons combine with positive ions, giving off light as they do so,
and then are blasted apart again.
The voltage supplied by the luminous tube power supply is
alternating; that is, it reverses itself many times every second. In
the US, current reverses itself 60 times per second for conventional
electrical systems and newer electronic systems reverse up to 20,000
times (or more) per second.
Each electrode takes its turn being the positive one, and as a
result, the glow is distributed evenly over the whole tube. If direct
current had been used, with one electrode remaining positive, the
situation shown in (a) and (b) of Figure 2 would occur. This
undesirable condition is avoided by the use of alternating current.
Figure 2: Schematic diagram of low pressure
gas-filled tubes. The top two show the distribution of the
positive column when direct current is used. When
alternating current is used (bottom), the distribution is
The reason for high voltage
Unless high voltage is used, the electrons will not be attracted
strongly enough to start the ionization process. Without ionization,
no current will flow and no light will appear. Hence, a high voltage
must be used. Transformers are built with secondary voltages that
vary from 2000 to 15,000 volts for signs. In cold-cathode ballast
systems, the secondary voltage is standardized at 750 or 900 volts.
Once the glow discharge has begun, however, less voltage is
required to keep it operating. In fact, if the original, high
starting voltage is maintained, the current becomes too great, and
the gas heats up excessively. The tube will consume a great deal of
power, become very inefficient, and its life will greatly decrease.
Therefore, some means must be provided for lowering the voltage after
the glow has started.
A special type of transformer, made especially for luminous tubes
called a high-leakage reactance transformer, performs this voltage
reduction automatically. The method used is described in detail in a
The electric current that passes through the luminous tube
determines the tube's brightness. If the current is too low, the tube
will lack brilliance; if it is too high, the tube will overheat and
have a very short life. The required operating current depends upon
the diameter of the tubing, the kind of electrodes used, the kind of
gas and its pressure. Each of these factors must be correct so that
the operating current is neither too small nor too large.
The current of neon signs and cold-cathode tubing is measured in
milliamperes. A milliampere is one-thousandth of an ampere, which is
the standard electrical unit of current. Operating currents in
standard signs run from 15 to 60 milliamperes. In cold-cathode
lighting operating currents may run up to 120 milliamperes.
The current through the tube is accompanied by the passage of free
electrons from the gas into the electrodes. The higher the current,
the more electrons will flow to the electrodes in a given time. Each
time that an electron hits the electrode, the energy with which it
hits is transformed into heat, and as a result, the electrode heats
up. If the current is excessive, the electrode will begin to
disintegrate, or even to melt. Hence, electrodes must be designed to
withstand considerable heat, although even with proper design they
cannot last if the operating current is too high.
The effect of gas pressure
We have already seen that a steady glow is possible only if the
gas in the tube is at a reduced pressure. The pressure is the force
with which the gas inside the tube presses against the glass wall of
the tube. Pressure might be expressed as pounds per square inch
(psi), as air pressure is often expressed. (Normal air pressure, or
"atmospheric pressure" is approximately 14.7 pounds per square inch
at sea level). Usually, however, pressures in luminous tubes are
expressed in terms of the height of the column of mercury which that
pressure will support.
Figure 3: Curve showing increase of current as
pressure is reduced (tube fed from constant-voltage source).
Note decrease of current when pressure falls below 3
At sea level, air pressure will support a column of mercury to
about 760 millimeters or 29.9 inches. The pressure of the atmosphere
is thus referred to as a pressure of 760mm. When the tube is pumped
out, the small amount of gas that still remains in the tube can exert
only a small pressure, compared with that of the air-filled tube. The
gas pressure in a luminous tube, for example, is usually between 3
and 20mm. The larger the diameter of tubing used the lower is the
pressure required. Vacuum pressure measured in luminous tubes, before
filling with gas, often goes down to less than 0.001mm.
The amount of current that will flow through the tube depends
largely on the pressure. To realize why this is true we must remember
that in a gas at low pressure, there are fewer atoms per cubic inch
than in a gas at higher pressure. That is, in the low-pressure gas,
the atoms are fewer in number and farther apart from one another. In
the low-pressure gas, therefore, the free electron has a longer
distance in which to get up speed before it hits a neutral atom. As a
consequence, when the free electron hits, it hits much harder, and
more free electrons are liberated than otherwise would be free, and
the whole action is much more intense. Hence, as the pressure is
lowered, the current will increase. This is the general rule for
pressures encountered in luminous tubes.
If, however, the pressure is reduced so much that a nearly perfect
vacuum forms inside the tube, then there are so few atoms available
that the current will decrease from sheer lack of electrons and ions,
even though the electron speeds are very high. Consequently, it is
found that, after a certain point is reached in lowering the
pressure, the current begins to decrease again. The effect of
pressure on current is shown graphically in Figure 3.
The effect of diameter of the tubing
Glass tubes for sign-lighting purposes usually vary between 7 and
15mm in outside diameter. These sizes are convenient to work with in
practical applications. In many countries outside the US, signs are
made with larger tubing, up to 20mm in diameter. In general, tubes
larger than 15mm in diameter are not used because the current
available from standard sign transformers is limited to 60
milliamperes (mA). This amount of current can light a 15mm tube at
standard pressure effectively. If the same current were applied to a
larger tube, the light would spread thinly throughout the tube,
nullifying its effectiveness as a light source.
In general, the smaller the tube used, the more brilliant the
light. To obtain the maximum brilliance with the larger sizes of
tubing, it is sometimes necessary to use slightly higher operating
currents. The smaller the diameter of the tubing, the higher the
resistance, and the higher voltage required to operate a foot of
tubing. Figure 4 shows the relationship between the necessary
voltage and the diameter of the tube.
The foregoing remarks apply to luminous tubes of the
nonfluorescent type of cold-cathode lighting. In fluorescent
lighting, large-diameter tubes are often used, up to 25mm outside
diameter in the cold-cathode types, and up to 54mm in the hot-cathode
One reason for the larger diameter of the fluorescent tubes is the
fact that the light is very intense, and the surface brightness of
the tube is reduced by spreading the light over the larger area. This
is particularly important in white tubes used for interior lighting,
because too high a surface brightness may prove annoying to the eyes
in interior installations.
The gases used for luminous tubes
No attempt will be made to go into details here concerning the
gases used in luminous tubes, because they are fully described in the
following chapter. In this introduction, only their electrical
characteristics will be discussed, in relation to other electrical
properties of the tubes.
Figure 4: The voltage per foot required to operate
tubing of various diameters filled with neon gas (no
The rare gases -- neon, argon, helium, xenon and krypton -- are
ideally suited for use in signs. The voltage per foot needed to
produce a suitably brilliant light is much less than more common
gases, such as nitrogen and carbon dioxide. But the gases with the
lowest resistance are not those that produce the most light.
Argon, for example, has a very low resistance, lower than that of
neon, but its light is comparatively weak. To obtain the advantages
of low resistance and good light emission, gases are sometimes
combined. Low-resistance gases are mixed with the good light givers
to produce a compromise having the advantages of each type of gas.
Most often, mercury vapor and a low resistance gas are combined.
The use of mercury
The most efficient and only practical way of producing many colors
requires mercury vapor and a carrier gas. Vapor differs from gas in
that a vapor can exist only as it evaporates from a liquid, while a
gas can exist of its own accord.
To obtain mercury vapor, therefore, it is necessary to insert
liquid mercury in the tube. As will be shown in later chapters the
light from mercury vapor isn't only useful for its visible component
but also for its invisible ultraviolet component. The ultraviolet
portion of the light can be used to energize chemicals known as
phosphors, which when excited radiate a whole spectrum of colors,
producing the majority of the wide array of colors used in signage
To start a current flowing through mercury vapor is not as easy as
with a gas, and for this reason a gas (usually argon or argon-neon
mixture) is mixed with the vapor to aid the current flow. This extra
gas also contributes somewhat to the light of the discharge, but its
main function is that of a current carrier. The mercury vapor, like
other vapors, is very much affected by changes in temperature; it
will condense as the temperature is lowered. Thus, the light will
become much more intense after the sign has had a chance to warm up.
In extremely cold weather, the sign may never come up to full
brilliance unless it is properly constructed. In installations
subject to extreme cold, special starting gases containing helium or
neon are often used. The heat of these gases or mixtures maintains
the mercury-vapor pressure. All these factors must be taken into
account in designing a sign for the particular job that is required.
In general, the smaller the diameter of tubing used, the higher the
resistance per foot and the hotter the tube will get, allowing the
mercury to vaporize more readily. In cold weather, however,
small-diameter mercury tubes are often found to fade out while larger
tubes remain bright. This is due to the higher heat losses of
Chemical effects inside the tube
The action of the luminous tube in producing light of the required
brilliance and color depends wholly upon the electrical ionization of
the gas within it. If the action could be restricted to a purely
electrical one, there would not be concerns as to the sign continuing
to function properly. Unfortunately, there are many possible chemical
actions which may start within the tube if special precautions are
not taken. These chemical actions almost universally lower the
efficiency of the tube. If any impurities, such as dirt, grease, or
impure gas, are left in the tube after it is sealed off, these
impurities, under the action of heat and electrical stress, become
chemically active. As they combine with one another, they may blacken
the glass, they may combine with the metal of the electrodes, or, if
the heat is intense, they may liberate gas inside the tube. If this
last possibility occurs, the unwanted gas will become ionized, and it
will give off light and usually excess heat which leads to further
release of impurities resulting in a tube that progressively gets
worse until failure. This accounts for the fact that neon tubes often
turn blue when they go bad. Many failures in tube making result from
insufficient care in eliminating the impurities during pumping and
filling. The gas inside the tube must be pure and the glass and
electrodes must be thoroughly clean if undesirable chemical effects
are to be avoided. Above all, the mercury must be the purest
Bombarding, an essential operation in tube making
Removing the chemical impurities is not so easy a task as may be
supposed. Although the electrodes, the glass, and the rare gas may
appear to be clean, they actually harbor many impurities that cling
to the surface or are in the glass structure itself and which cannot
be removed by merely pumping them out. The best way of getting rid of
them is to heat the tube before or during the time it is being pumped
out. The heat drives the gas and other impurities from both the metal
and glass in the form of gas or vapor. The vacuum pump removes these
gases, and if the heating process is kept up long enough, the vast
majority of all of them are removed. The tube may then be filled with
rare gas and sealed off.
The simplest way of heating the tube and electrodes is by passing
a current through the tube while it is still on the pump. This can
only be done when the air pressure has been reduced sufficiently to
allow a heavy current to flow. This heavy current, usually much
higher than the operating current, will pass through the low-pressure
air in the tube, giving off much light and heat in the process. The
light serves no purpose except to show that the bombardment, as it is
called, is actually taking place. The heating has the desired effect
of ridding the tube of its impurities. Bombardment is a most
important and exacting process. A later chapter has been devoted,
therefore, to explaining what it is and how it should be carried out.
Electrodes, tube life and sputtering
The electrode has the task of carrying current from the power
supply wires to the rare gas. Because it is continually subjected to
the bombarding of electrons and ions, it heats up, and therefore must
be designed to withstand heat. Since the metal is hot, it is highly
active chemically, and may combine with gases or impurities within
the tube. But by far the greatest difficulty with electrodes arises
from what is known as "sputtering." Sputtering occurs when the
electrode, under the impact of the heavy ions, flies to pieces bit by
bit. The metal of the electrode gradually flies off and coats the
inside of the glass tube. This effect in itself causes no harm, since
the blackening caused by the metal deposit is confined to the ends of
the tubes near the electrodes. Eventually, of course, the entire
electrode is consumed by the process, but since the action is very
slow, the electrode will nevertheless last for normal life. However,
sputtering is accompanied by a decrease of gas pressure in the tube.
This loss of pressure eventually makes the tube inoperative.
The sputtered metal from the electrode absorbs some of the fill
gas in the tube. As the gas is absorbed, the pressure in the tube is
reduced leading to what is called "hardening" of the tube. The
reduced gas pressure means there are fewer gas molecules in the tube
and the electrons and ions can travel greater distances before
hitting each other or a gas molecule. These particles therefore can
build up significant speed before they impact the electrode. The high
energy impact on the electrode causes a good deal of heat in the
glass near the electrode. Eventually the glass around the electrode
will heat until the relative vacuum in the tube sucks in the hot
glass, causing the tube to fail. In the early days, this sort of
trouble was very common; in fact, the short life of tubes (due to
sputtering) was one of the greatest hindrances to the commercial
introduction of tube lighting.
The logical solution to the sputter problem is to build electrodes
that will not sputter under ordinary conditions. Some sputtering will
always occur, and the life of the sign is thus always limited. But if
the sputtering action is controlled, the life of the tube will be
predictable, and maintenance guarantees and costs can be figured
First, electrodes must have a large area exposed for the
dissipation of heat. If an electrode runs cool, it will sputter much
less violently than if it is hot. Secondly, the electrode must be
made of the kind of metal that will resist the sputtering action.
Special metals and common ones have been tried. The best electrodes
to use for each gas, each pressure, each operating current, and that
are affordable to the industry, are known and have been more or less
standardized. Third, manufacturers are now making electrodes with
ceramic collars at the forward end of the electrode. The ceramic
collar won't break down as readily as the metal of the rest of the
electrode and so acts as a buffer to prevent much of the sputtering.
Finally, the accurate filling of the tube with gas is important since
underfilling the tube results in too little a gas reservoir to
counter the gas lost to sputtering. Too low a pressure also acts to
encourage sputtering since a larger distance between molecules allows
molecules to pick up a great deal of speed before impacting each
other or the electrodes.
The electrodes can be treated chemically to reduce sputtering by
the application of an emissive coating before they are put in the
tube. Special patented processes are sometimes used for this purpose.
The coating provides a more ready source of free electrons than would
the metal alone. If properly processed the tube will start and run at
a lower voltage thus reducing the electrical stress on the tube as
well as damage due to sputtering. This treatment is sometimes
absolutely necessary to ensure reasonable life for a sign. Treated
electrodes are especially necessary for helium tubes. Special
mechanical features are also incorporated in electrodes to reduce the
tendency to sputter. All these features are treated at length in the
section on electrodes in the next chapter.
Mechanical requirements of the tubing
The maintenance of the proper gas pressure can be threatened by a
more direct action than that of sputtering. If the glass envelope is
not completely vacuum tight, air will leak in, and the sign will soon
go dead. To assure complete air tightness, the glass should first be
mechanically strong and thus not subject to cracks or other breakage.
This requires great care in glass blowing, because if the glass is
cooled too quickly or unevenly, it will be left in a very brittle and
fragile state. Unlike most blown glass produced for other industries,
neon sign tubing is not normally stress relieved after the bender
works the hot glass. This leaves strain in the glass that can lead to
breakage of the signs if exposed to rapid temperature fluctuations or
rough handling. Particularly for signs installed out of doors, such
strains within the glass will cause trouble, since the sign is then
subjected to extremes in temperature.
The second requirement for a vacuum-tight tube is concerned with
the lead-in wires that connect the transformer wires with the
electrodes. Except for these wires, the whole wall of the tube is
made of glass. The leads must necessarily be made of metal, to
conduct the current. To make a good joint between glass and wire, the
glass must "wet" the wire, that is, adhere to it firmly. Molten glass
and wire do not readily cling to each other in this way unless the
wire is copper or copper coated.
When wire and glass are heated, as when the wire is sealed on the
glass or during the operation of the sign, both glass and wire
expand. If the wire tends to expand more than the glass, it will
press against the glass with great force. If this force does not
crack the glass at once, it will subject it to a great deal of strain
which may crack it later, or at least lead to small leaks around the
seal. It is important, therefore, that the glass and the wire expand
at the same rate as they are heated. If they do so, no internal
stress will occur, and the joint will be strong. In practice, a
special alloy wire ("dumet" wire) is used for soft glass. This wire
has a coefficient of expansion very nearly equal to that of the glass
over the entire temperature range to which it will be exposed in
manufacture and service. The seal will remain vacuum tight and
serviceable if it is thus properly made. For borosilicate glass --
Pyrex® -- tungsten is used for the metal-to-glass seal
and often a series of transition glasses are also used that more
closely match the tungsten expansion rate.
Simple as a luminous tube is, it is potentially a very dangerous
piece of apparatus. When properly constructed and installed, it
presents less danger than an ordinary lamp socket. But if proper
precautions are not taken, the high voltage used is definitely
dangerous. Since the current is limited to 30 or 60 milliamperes in
most cases, the neon sign usually cannot produce a shock of
sufficient force to kill an adult. But the shock from a neon sign is
powerful enough to overthrow someone, and the resulting fall may
easily kill or cripple him or her. Therefore, the possibility of the
public coming in contact with the high voltage is far too great a
danger to be allowed.
The installations for interior lighting using cold-cathode tubes,
especially high-voltage systems, must be made and designed with
special care, since the possibility of accidental contact or fire
hazards is greater than in exterior sign displays. For such
installations it is important that the manufacturer comply with the
rules and regulations of governing bodies such as the National
Electric Code, or private regulators such as the Underwriters'
Laboratories, as well as the local authorities, and follow the
procedures outlined by them. In this way, these installations can be
made and installed with as little danger as the installations of any
other electrical appliance.
The precautions which must be taken involve the thorough
insulation of every metallic part in the high voltage circuit. Simply
putting the wires and electrodes out of reach of the prying fingers
of children and innocent bystanders is not enough, because the wires
may come in contact with metal moldings, showcase findings, etc.,
which will either conduct the high voltage directly to the
unsuspecting public or will result in arcing to a nearby conductor
resulting in a possible fire. To avoid this, the industry has
evolved, with the help of the Underwriters' Laboratories, a very
complete set of insulating fittings and connecting wires that
completely shield the high-voltage wiring not only from the public
but from the weather as well. Protection from weather is essential in
The wires that lead from the transformer to the electrode leads
are thoroughly coated with a high-resistance insulation capable of
withstanding more than 15,000 volts. Special insulators of ceramic,
Pyrex glass, or plastic are used to connect the electrode and its
wiring to the high voltage cable. The connection of the high-voltage
cable to the power supply is likewise protected by a ceramic bushing.
The entire high-voltage circuit is thus completely enclosed with
insulation from start to finish. When so protected, it is actually
far less dangerous than the exposed lamp socket in the home.
Operations involved in making a luminous tube
By way of summarizing the foregoing brief introduction of the
fundamentals of luminous tubes, the following outline of the
construction methods is presented.
A large workbench fitted with a fire-proof covering is required
for laying out the patterns and for the actual work of glass bending.
A supply of gas and sometimes a means for raising or lowering its
pressure is required. An air supply is necessary, usually from a
low-pressure, high volume air compressor. A variety of gas burners,
or "glass fires," connected to the gas and air supply, is used in
heating the glass tubing to the plastic (soft) point, so that it can
be bent to the desired shape.
After bending the tubing to the desired shapes, the tube has
electrodes sealed onto the two ends and the tube is connected to a
pumping system called the neon manifold. A well-maintained pumping
system, capable of drawing a good vacuum in the glass tube is needed.
Also needed is a supply of inert backfilling gases such as neon and
argon (or a custom mix of these two for cold-weather work). In
addition to the gases, liquid mercury and a means for injecting it
into the tube is required. The manifold should also be equipped with
several indicator gauges of good quality for measuring both the
vacuum level in the tube before backfilling, and the accurate
pressure of the inert gas fill. A testing coil for testing for vacuum
leaks is also handy for troubleshooting both tubes and the manifold.
Finally, supplies of glass of various diameters and appropriately
sized electrodes are needed.
Constructing a luminous tube sign
A neon sign usually starts out as a sketch. The sketch must first
be made into a full-sized drawing in "tube form" showing the sign as
it would appear in bent glass. Various methods can be used to enlarge
artwork to the appropriate size. Most neon patterns are still done by
hand, although computer methods and optical projection methods are
also used. The image, whether it is of letters or graphics, is then
reversed and transferred onto heavy paper or heat-resistant cloth.
The pattern is reversed because the bender actually works from the
back of the sign as the hot glass is shaped onto the pattern.
The glass is then heated in the glass fires and the various bends
are made. If the sign is made up of long lengths of tubing, parts may
be made separately and the pieces spliced together later. Two
electrodes are spliced onto the ends of the tube, one on each end.
Normally one end has a tubulated electrode, which is an electrode
with a tube in it used for evacuating the air and backfilling the
The tube is now attached to the neon manifold via the tubulated
electrode. The main vacuum stopcock is opened and the pressure in the
tube is reduced to the point where the bombarding transformer can
form a discharge through the tube. The heavy bombarder current
through the low pressure air quickly heats up the tube. Before
turning the bombarder off, the glass heats to about 450°F and
the metal electrodes heat to a bright red color. The bombarder is
turned off and the main vacuum valve is opened and all the vaporized
gases are evacuated.
Figure 5 A schematic layout of a box-type neon
sign, showing relative location of transformer, mounting
parts, cable, tubing,etc.
After thorough evacuation, which is usually determined by the
vacuum gauge, the main vacuum valve is closed and the inert gas is
carefully ladled in, filling the tube to a pressure dependent on the
diameter of the tube. The tubulation is then tipped off with a small
hand torch and the sealed tube is ready for aging.
Aging refers to the initial burning period of the tube during
which any trace impurities may be absorbed, allowing the tube to age
up to its proper color and brilliance. During the aging process,
small levels of impurities are cleaned up chemically, although this
is not a substitute for proper processing before tipping off the
tube. Aging should not take more than a few minutes. If so, the
bombarding and pumping processes should be checked.
The tube is now operational but portions of the tube called
crossovers, the connections between letters and words that aren't
intended to be seen, can be painted out using light blocking paint.
Mounting the tube is a process that largely depends upon the
particular installation. However, a typical external neon
installation is shown in Figure 5. In this type installation a
background of metal is made against which the tube glows and which
serves to emphasize each letter. The box or housing also serves to
mount the transformer. Standoffs are mounted on the background which
hold the glass tube at several points. Two recessed housings or
bushings are provided for insulating the electrodes from the metal
sign box. The high-voltage cable that connects the electrodes to each
other or to the high-voltage power supply is wired internally with
care not to run too close together or too close to the grounded metal
box. The whole sign is assembled at the shop and shipped with or
without the neon installed to the job site. At the job site the sign
is secured into place and the primary wiring can be connected to the
Flashers, time switches, and remote switches can be installed to
the primary wiring of the sign. Depending upon the care with which it
has been designed and constructed, this type of sign should give
three to five years of continuous operation before requiring any
The maintenance problem
The limited life of even the best constructed sign should not be
forgotten by the sign maker who intends to make money in this
business. The sign maker must know how long the signs can be expected
to last under various conditions, and the sign must be sold with this
in mind. The customer must understand the necessity for periodic
maintenance and replacement of worn parts. Sign prices should be
calculated to include maintenance for a certain period of time. One
of the most difficult business decisions is how long to warrantee a
sign and how to handle relations relative to the service on signs.