February 1931 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
Galvanometers are the basis for most analog type meter movements
- remember those things? They work by having the current flowing through a fine wire coil armature generate
a magnetic field that displaces an attached needle. Most galvanometers respond nearly linearly to applied
current, and therefore are used in voltmeters and ammeters with simple resistor series and parallel
configurations. The current-squared galvanometer, on the other hand, responds roughly linearly to power
values. It can therefore be used in a transmitter output monitoring circuit, for instance, or within
amplifier interstage coupling circuits to indicate peak coupling values while tuning. Yes, a linear
ammeter could be used, but its needle deflection in terms of power would be nonlinear and could make obtaining fine adjustments
readings difficult to discern.
The Neglected Current-Squared Galvanometer
Some Suggestions for Its Use
By Paul E. Griffith, W9DBW*
One instrument that is rarely seen in an amateur station is the current-squared galvanometer. On
first thought the scarcity of such instruments in amateur stations might be attributed to their relatively
high cost; but after analyzing the situation one sees that it is not so much the cost but rather the
lack of appreciation of their many uses which has kept them from being more popular.
Fig. 1 - Schematic
S - Source of alternating current
G - Current-squared galvanometer
Z -Impedance being measured
R - Non-inductive variable resistor
The current-squared galvanometer is nothing more than a thermocouple type meter which has a scale
calibrated in 100 evenly spaced divisions of arbitrary value instead of the usual logarithmic scale
common to thermocouple type ammeters. The deflection of the pointer is directly proportional to the
square of the current and the scale reading, therefore, is directly proportional to the power in the
circuit. Standard types of current-squared galvanometers require 115 to 125 ma. for full-scale deflection
and have a radio-frequency resistance of 4.5 ohms. The meter itself, therefore, dissipates less than
one-tenth watt at full-scale deflection. Because of its comparatively small current capacity and low
resistance, caution must be used in its handling. This applies particularly to its use in making measurements
of radio-frequency power, especially in transmitters.
An instrument such as the Weston Model 425 or Jewell Pattern 67 thermo-galvanometer is a most versatile
aid in making measurements of alternating currents at practically any frequency used for communication
A Neutralizing Indicator
As a device for use in neutralizing power-amplifier circuit, it is excelled only by much more expensive
and elaborate layouts. It is used in place of the usual flashlight bulb and pick-up coils and in exactly
the same way. A coil of one or two turns of wire is connected to the terminals of the meter and placed
in inductive relation to the plate tank coil of the tube being neutralized. It is surprising how accurately
and quickly the minimum current adjustment can be found. One is sure that the set is properly neutralized
because there is the visual quantitative indication provided by the meter scale as a basis for judgment.
Again, the meter will cause less detuning effect because it needs less current to operate it, the full-scale
deflection requiring only 125 ma. or less as compared to the usual 300 ma. necessary to light a flashlight
bulb to full brilliancy.
After all stages have been neutralized it is a simple matter to loosely couple the meter to the output
tank circuit and go over the transmitter tuning step by step, tuning for maximum scale deflection of
the meter always. The meter deflection is very nearly proportional to the power output, because of the
current-squared feature, and the fixed resistance of the pick-up circuit. (P = I2R.)
One of these meters with its pick-up coil will take the place of several r.f. ammeters in tank circuits,
thus reducing the tank circuit resistance. It will also be available for other uses, such as those mentioned
below. By making inexpensive fixed mountings - one beside each plate coil - the meter may be easily
supported, enabling one to be sure that any change in deflection is caused by a change in the transmitter
and not a change in position of the pick-up coil.
Other Audio- and Radio-Frequency Applications
Fig. 2. - Graph of R vs. % Modulation
Other uses of the galvanometer are as a volume indicator; as a current indicator in all a.c. measurements;
as a resonance indicator in wavemeter circuits; and as a means of measuring percentage of modulation.
A special transformer must be used with the meter if it is to be utilized as a volume indicator in
conjunction with audio-frequency amplifiers. This may be made from an Amertran Type 854 50-henry choke,
or any similar choke with enough room in the core window to permit 80 turns of number 18 d.c.c. wire
to be wound over the coil. The large winding is then connected to the primary of the output transformer
of the amplifier and the meter is connected to the small 80-turn winding.
It will be noted that any transformer which has a secondary winding of the proper impedance to match
that of the meter (4.5 ohms) can be substituted for the one described above, although it should have
a high primary impedance in order to reduce the shunting effect on the plate load impedance. The special
transformer previously referred to reduces the plate load impedance of the final stage of the amplifier
by about 18.8%, which is certainly far from negligible. It would be better to connect a coupling transformer
of the proper ratio to the volume indicator, so that the tone quality of the amplifier would remain
unaltered. This, and the fact that at least one stage of amplification is needed to operate it, are
the only objectionable features of the device.
There are numerous experiments and measurements found in the books listed by the QST Book Department
in which a galvanometer that will measure alternating current of almost any frequency will prove valuable.
The current-squared type is not too large to be used in any of these experiments. Of course, a shunt
may be used if it proves to be too small.
A handy method of measuring impedance which is not found in many books uses some sort of a.c. measuring
instrument. The meter described is very good in this case. To measure the impedance of a coil or condenser,
or a combination of both, it is only necessary to apply an alternating voltage of constant amplitude
and frequency to it and to measure the current flowing through it. A non-inductive variable resistor
is substituted for the impedance unit and adjusted until the same current flows. The value of the resistance
in ohms is the value of the impedance in ohms. The circuit of the arrangement is that of Fig. 1.
Perhaps the original use to which the current-squared galvanometer was put was that of resonance
indicator in the absorption-type wavemeter . It is still used on long-wave wavemeters; but the heterodyne
frequency meter and the piezo-oscillator have displaced such instruments in modern short-wave stations.
As a means of measuring the percentage of modulation, the galvanometer is by far the most rapid instrument,
although it is not accurate to a very high degree. A full explanation of the method used in such measurements
appeared in QST for May, 1930, on page 48. To make the employment of this method easier and faster,
a curve is given in Fig. 2 which was computed from the formula given in the article referred to above.
R is the same as the R in the article; the ratio of galvanometer reading during modulation to that with
* State University of Iowa, Iowa City, Iowa.
Posted August 24, 2016