March 1935 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Resistors, probably the most common
electronic components in existence, have undergone significant evolution since first
being mass produced in the late 19th century. Amazingly, less than two centuries
have passed since the concept of electrical resistance was first published in 1827
by Georg Simon Ohm,
a German physicist. As with most products of the era, and well into the 20th century,
resistor manufacturing involved a degree of human labor. Resistors (and capacitors
and inductors for that matter) found in your grandparents' old vacuum tube radios
were most likely measured and sorted, and the colored value markings painted by
the hand of a human worker. This "How Resistors Are Made" article from a 1935
issue of Radio-Craft magazine provides a look at the early manufacturing process
long before salt-grain size surface mount resistors. Note the photo showing an
operator manually painting color code dots on the resistor body. One of America's earliest and largest resistor manufacturers
was based right here in my adopted hometown of Erie, Pennsylvania. Here is a short
feature I wrote on
Erie Resistor Corporation.
How Resistors Are Made
Manufacturing operator applies colored paint for value and
The radio beginner has no doubt wondered how resistors are made.
This interesting article tells how the different types are manufactured, including
a new method.
Resistors are now made by an entirely new process, by a well-known manufacturer,
developed over the past six years as a result of intensive study of the different
classes of resistors to eliminate the drawbacks of the general methods heretofore
A study of resistors shows that they fall into three classes, namely: wire-wound
type, carbon-coated or film type, and carbon-composition molded type.
The first class is wire wound, and from a cost standpoint can be disregarded.
The second class is the so-called carbon-coated type, in which a thin film of
carbon is placed on a glass or porcelain rod or tube. In some cases this is spiraled
to vary the resistance. Briefly, this type has generally been discarded today, and
spiraled units are not being made in important quantities now. The film of carbon
is very thin, and therefore, very fragile and unsafe. Mechanically such a structure
is undesirable. Injuries easily occur; adhesion of the film to the supporting surface
is difficult to control; contact is fragile; and transfer of heat energy depends
on mechanical limitations of the whole assembly. The current-carrying capacity must
necessarily be a function of the area or cross section and when the resistance is
increased solely by cutting down the thickness of the film (already very thin),
naturally the current-carrying capacity is impaired, and the current density increases
tremendously. This gives rise to overheating, localized stresses and leads to ultimate
failure and bad characteristics.
The third class is the so-called carbon stick or composition type. It consists
of a mixture of a very small percentage of conducting material (carbon) and an insulating
material molded under relatively low pressures. Practically it really represents
a number of semi-round carbon pebbles which touch each other with a point contact.
Therefore, the paths and areas through which current may travel are reduced greatly
by the large bulk of insulating particles, and this to a tremendous degree further
by the point contact condition.
The current density must be considered microscopically, and consequently has
heretofore not been stressed to the extent its importance warrants. A study of micro-photographs
will bear this out. It shows that the structure of such units is not uniform and
contains many voids, irregularities and inclusions. It is seen that the mass is
not solid but porous and contact is "point" contact only. Therefore the current-carrying
cross section is microscopic, and the current density must of necessity be very
high at such points or areas. This has been proven by studying under high-powered
microscopes the action of the units under both normal and excessive loads. Glowing
points of light were observed proving the intense microscopic current density. Naturally,
such points would tend to alter their characteristics both physically and mechanically.
Further study was made of resistance values under mechanical loads, and the changes
noted under such loads were to be expected in accordance with the above facts.
With these considerations in mind a resistance was developed which technically
and practically would overcome these objections. First, instead of a background
material of very high insulating value, a background material which is in itself
a resistance material has been substituted. To vary this material and o get the
desired resistance values another resistance material of lower value is introduced.
The entire mass, after it has been reduced to absolute uniformity is then subject
to tremendous pressure and under such pressure extruded into rods.
Consider then the result. Instead of a very small percentage of the cross section
being of a current-carrying material, the entire cross-section is current carrying.
Further, the cross-sectional area is not composed of a great number of voids and
a relatively small number of points of contact, but is microscopically one solid
uniform compact current-carrying mass. This is the result of the tremendous pressure,
the method of extruding, and the composition of the material itself. In fact, these
new units are so uniform that they resemble the micro-photographs of a section of
a gun forging. This current-carrying area is large and non-microscopic, and this
in itself explains many other results as shown in the succeeding paragraphs.
A study of these units under mechanical loads, shows that the resistance value
remains constant. This is a most significant fact. Microscopic study under normal
and excessive wattage shows that "light" points, or points of excessive microscopic
current density do not exist.
The Results of Tests
The method of attaching ends to these units has also been subject to a similar
technical study and development. It was desired to secure a uniform sound area of
contact, and then to place on this area of contact a metal surface which in itself
has sufficient strength and rigidity to be positive in its action. The coating of
the ends of the resistor with fine particles of metal produced the first consideration.
The second requirement was met by placing on the end of the resistor and over this
metal-coated surface a solid piece of metal in the form of an end cap to which end
cap was integrally attached the pigtail. In this fashion a much sounder terminal
construction was arrived at than in most instances.
The physical appearance of the unit bears out these facts. One finds a solid
extruded mass., homogenous, and rock-hard which is a conductor throughout its entire
body, has a smooth velvety finish, which in itself is ideal for heat radiation.
The diameter and length is uniform.
The watting ratings are exceptionally conservative for the sizes have been kept
standard (to the usual type resistor dimensions) rather than reduce the bulk in
keeping with the greater wattage dissipating properties of he unique conducting
These units have been in actual existence and on test for some years and have
been in their final form for more than a year.
The first consideration in testing was to apply load. All loading was applied
initially at double wattage, with no effect on the value of the unit. Increased
load was applied in the presence of elevated temperatures, still the units were
ale to show no change in value. Tests were made on an intermittent basis and the
loads were varied from less than normal to more than double load. Tests were carried
out by many different groups., and were highly satisfactory.
One of the most recent tests shows that not a single unit dropped in value under
various loads up to double wattage and voltage up to 880 volts and greater. Variation
of load of any one unit was less than 2 per cent, but the majority of the units
were fractions of 1 per cent. In fact, loads in excess of 1 watt were placed on
the 1/4-watt units.
These units were tested also under usual humidity conditions and found to be
without change. They were then placed in water for varying periods of time, and
again satisfactory results. Further, these units have been subjected to any unusual
test; such as putting them directly into water; into boiling water; and into live
steam with satisfactory results.
Posted October 20, 2023
(updated from original post on 10/4/2016)