July 1933 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
While reading through this article on copper-oxide rectifiers,
I am once again reminded of how much we take for granted the
conveniences of electrical test equipment on today's shop benches.
The advent of FET-input multimeters was a huge step forward
because the meter input impedance is so high that it has practically
no impact on the circuit being measured. Prior to that, most
simple meters drew their power from the circuit under test,
thereby altering the true value of current or voltage being
measured. Of course there were vacuum tube voltmeters (VTVM)
with high input impedances, but few hobbyists or laymen could
afford them. This piece reports on how the advent of a non-tube-based
rectifier permitted alternating current (AC) measurements to
be made by DC-driven d'Arsonval meter movements so as to not
excessively load the circuit being measured. Rectox meters had
the rectification components and calibration built in, and while
still more expensive than circuit-driven meters, were much more
affordable than other types. Today, of course, you can get a
free DMM with more accuracy and greater functionality than any
1930s era multimeter at
Freight just by handing the clerk a readily available coupon.
Applications and Characteristics of Copper-Oxide Rectifiers
*Meter and Instrument Section, Westinghouse Electric and
The copper-oxide rectifier is used so extensively in modern
test equipment, and is so little understood, that the editors
are pleased to present this excellent discussion of the uses
and limitations of copper-oxide rectifiers.
An actual photograph illustrating the size
of the rectifier unit.
The practical measurement of alternating currents has, heretofore,
been made by three types of instruments: the electrodynamometer,
the repulsion or attraction iron vane type and thermo-couple-d'Arsonval
type. The electrodynamometer type has a system of stationary
and moving coils without iron in the magnetic circuits; the
repulsion iron-vane types have a stationary coil, a movable
vane and a stationary vane. Some types of this movement consist
of a stationary coil and a single, movable attraction iron vane.
In the thermo-couple-d'Arsonval type - a thermocouple is heated
by the alternating current and the resulting thermo-emf is measured
by a d'Arsonval instrument. Now we have a fourth, practical
A. C. instrument known as the Rectox type. It consists of a
copper-oxide rectifier and a d'Arsonval type of instrument.
The alternating current is rectified and then measured by the
ordinary direct-current d'Arsonval instrument.
Fig. 1 - A simple sketch showing the connections
and calibration of Rectox instruments.
The energy consumption, or the power required to operate
the pointer; in the previous types of alternating-current instruments
is much more than that required for direct-current instruments.
This is because, in a direct-current instrument, the magnetic
field is supplied from a strong permanent magnet, permitting
a comparatively small current in the moving coil. The higher
energy consumption of alternating current instruments has been
an application handicap for a long time, especially, where the
energy consumed by the instrument would seriously change the
circuit conditions, particularly in radio measurements. The
Rectox instrument has successfully solved this problem by embodying
the high sensitivity features of the d'Arsonval type in an alternating-current
Fig. 2 - A curve showing the relation between
the resistance of and the current through Rectox meters.
The rectifier used in the Rectox instrument is a product
of Westinghouse engineering and research. It is specially designed
for instrument use, the requirements of which are considerably
different from the usual well known battery charging applications.
The rectifier units are plates of copper which have been oxidized
on one side. Copper, when oxidized on its surface, has the peculiar
property of rectification, allowing current to flow much more
readily from the oxide to the copper, than from the copper to
the oxide. The copper plates are assembled and held firmly in
place, under constant pressure, by a sturdy clamp, and the entire
unit is impregnated to seal it from moisture and corrosion.
It is interesting to note that the size of these rectifier units
is considerably smaller than the usual battery-charging unit.
The relative area of the copper oxide plates greatly affects
the performance of rectifier instruments; the proper area of
plate has been determined after exhaustive research.
The assembly of copper plates is made to give full-wave rectification
for all instrument applications. This is accomplished by assembly
of the plates into four sections in reverse order and connecting
to the instrument as shown in Fig. 1.
The Rectox instrument has certain characteristics which makes
its application to measurement of alternating currents somewhat
critical. For this reason its operating characteristics should
be carefully considered in any application for measurement purposes.
Front view of the new instrument.
Rear view showing the rectifier.
This class of instruments differs from the usual alternating-current
types in that the torque and deflection are proportional to
the first power of the current. Therefore, it measures the average
value and not the effective value of the alternating-current
wave. The scale is, however, calibrated to read effective, or
root mean square, values of a pure sine wave. Consequently,
such instruments read correctly only on sine waves and have
serious errors on other than sine wave forms. These errors can
be compensated for in readings, provided the wave form is known
from which a correction factor can be applied; or, the instrument
may be calibrated on the wave form with which it is used.
Fig. 3 - Curve showing the relation between
the resistance and the temperature of copper-oxide rectifiers.
The resistance of the Rectox is a function of the current
flowing. This characteristic is shown in Fig. 2. When a rectifier-type
milliammeter is connected in a circuit, it affects the circuit
conditions because of its added resistance, like any other instrument;
also the changes in the circuit depend upon the value of current
passing. This disturbance of the normal circuit must be recognized
if the milliammeter resistance is a large percentage of the
total circuit resistance. If the circuit resistance is relatively
high, then this change will result in negligible effects. The
instrument always correctly indicates the actual current passing
through the circuit; but the magnitude of the current may depend
upon the non-linear value of the instrument resistance. (The
actual resistance of the Rectox varying with current. - Editor)
The readings of Rectox instruments are quite free from frequency
errors. The reading may be expected to decrease about 1/2 per
cent per kilocycle up to 35,000 cycles, where different conditions
occur. Because of capacity effects, this type of instrument
is not recommended for radio-frequency measurements. It is,
however, reasonably accurate throughout the audio frequency
The effect of current upon the resistance of a rectifier
unit has been previously discussed, but the copper-oxide unit
also has the property of changing its resistance with temperature;
and, furthermore, the amount of change due to temperature depends
on the amount of current passing. Temperature resistance curves
are shown in Fig. 3. We have, therefore, a very complex .relation
between current, temperature, and resistance. As a result of
these conditions, a great deal of skill is required in designing
a Rectox instrument to prevent errors arising from temperature
Fig. 4 - An interesting curve showing the
relation between the efficiency and A. C. input.
Tests show that the effective resistance of a copper-oxide
rectifier decreases as the temperature increases. Therefore,
if a rectifier instrument should be used as a low-range voltmeter,
without suitable temperature compensation, the voltmeter might
read as much as 20 or 25 percent high at a temperature of 40°C.
It is for reasons of this kind that little success has been
met in trying to adapt Rectox units to standard direct-current
instruments. Much better results have been obtained by use of
specifically designed combinations of instrument and rectifier,
in which proper temperature compensation has been developed.
Fig. 5 - Another curve, which shows the relation
between the efficiency and temperatures of copper-oxide rectifiers.
Note the variation with current.
Like all devices of its kind, the efficiency of the copper-oxide
rectifier is less than 100%; in other words, if 1 milliampere
A.C. is passed through it, the resulting rectified current available
for operating the indicating instrument is usually 8/10 of a
milliampere, or less. The typical current efficiency curve for
a rectifier instrument is shown in Fig. 4. Furthermore, the
current-efficiency ratio (D.C. current output divided by A.C.
current input) is affected to some extent by temperature and,
also, by the absolute value of current flowing. Fig. 5 shows
this characteristic. This again results in a complex situation
involving temperature, current, and efficiency; and a simultaneous
study of all of these variables is the only means by which errors
from temperature variations can be minimized. For example, with
certain values of current flowing, the efficiency of an uncompensated
rectifier instrument may drop from 80% to 75% during a 40°
C. temperature rise. This would result in lowering the calibration
of the instrument 6% at the higher temperature if no steps were
taken to secure temperature compensation.
The majority of the above discussed errors, characteristic
of rectifier instruments, can be minimized by careful design
and by taking advantage of the opposite effects of certain errors.
However, it is important that the instrument be properly designed
to operate with the copper-oxide rectifier. It is therefore,
not advisable to try to apply a copper-oxide rectifier to an
existing d'Arsonval instrument which has not been designed for
this application unless changes are made to provide proper moving
coil resistance, temperature compensation, and swamping resistance.
If the errors are properly cared for in the design of a complete
rectifier instrument, reasonable accuracy can be obtained.
The chief advantage of. a rectifier instrument is in
its high sensitivity. By use of the rectifier principle, alternating-current
voltmeters may be made with a very high resistance per volt.
Standard voltmeters are available in ratings as low as 4 volts
with a sensitivity of 1,000 ohms per volt, 1.5 volts with 2,000
ohms per volt, and even 0.5-volt with 5,000 ohms per volt. Below
four volts, rectifier voltmeters should have a resistance of
2,000 ohms and, better still, 5,000 ohms per volt, in order
to properly compensate for the errors discussed above. Milliammeters
and microammeters of low ratings are also available.
Rectifier instruments are rapidly finding their place in
the radio field for the measurement of such quantities as output
of amplifiers and oscillators and power level indicators. The
user of these instruments should bear their characteristics
in mind, particularly their accuracy, when used under various
conditions. Rectifier instruments are a valuable contribution
to the science of radio and they are continually finding new
uses in this rapidly advancing art. Possibly the further developments
in research and engineering on these instruments will tend to
minimize their present errors and make them still more useful.
Posted July 28, 2015