And even more Laboratory
Test Equipment!
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Foster
Portable Potentiometer Model 3155
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Before the days of multimeters
it was not a simple task to measure a voltage, resistance or
current very accurately. The usual way was to use a bridge where
a given voltage for example was set up within the measuring instrument
and the unknown voltage compared with this. If you examine the
dials shown below and read the instruction given on the lid you'll
see how measurements were done. The key to successful measurements
was of course the availability of an accurate voltage source
and if you wanted really accurate measurements temperature entered
into the equation... hence the thermometer clipped in place on
the front panel. Nowadays, all one needs is a cheap digital multimeter
and reasonably accurate readings can be made in seconds. |
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A metal-cased scientific
instrument donated by my friend Mike |
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Cambridge
Instruments Potentiometer
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A wooden-cased scientific
instrument also donated by Mike who is trying to rationalize
his collection of scientific instruments.
These things were the only solution
when one wanted to know the value of a voltage to more than a
couple of significant figures. With the advent of digital meters
these sort of things have been relegated to the scrapheap!
It's certainly not pocket-sized
as it weighs in at around 25 pounds. |
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General
Radio Wavemeter, Type 558-P
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This beautifully made
equipment, dating from 1928, was built for use by Radio Amateurs
by General Radio in the USA
The pine box accommodates the
wavemeter with its five plug-in coils, one of which, for low
VHF, is merely a metal loop and its calibration chart. |
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Below: The wavemeter with
one of the five coils fitted |
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In order to use the wavemeter
one must be able to detect its effect, either on a receiver or
a transmitter.
In conjunction with the chart
shown above one can then ascertain the frequency being measured.
This is a passive device and
works by absorbing energy at the frequency to which it is tuned.
If adjusted when closely coupled to an antenna, signal will be
sucked away as it were, leaving a dead spot. If positioned next
to an oscillator coil it may stop oscillations.
A similar useful device is called
a "Grid-dip meter". This is an active device employing
an oscillator and a small meter. When brought close to a tuned
circuit, resonance is shown by a dip or kick in the meter deflection. |
Here's an advertisement
in QST dated 1929
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Page 14 has the Type 558 Wavemeter |
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This is the W1117 a WWII wavemeter made for
the RAF
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This is an interesting
equipment as it uses a couple of ancient battery valves, VW48
and VW36 (RAF designations gave the second letter as the equipment
type.. "R" for "Receiver", "T"
for Transmitter, "W" for "Wavemeter" etc.),
and clearly (to me anyway) has a design which looks much earlier
than the date of 1941 which is to be found on its 500uAmp meter,
which plugs into a socket on the front of the panel.
Coverage is:- 125Kc/s-250Kc/s;
250Kc/s-500Kc/s; 500-1000Kc/s; 1000-2000Kc/s; 2-4.4Mc/s; 4.4-10Mc/s
and 9-20Mc/s. In terms of wavelength this represents 2,400metres
to 15metres over its seven selectable bands. |
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The thing was operated from
built-in batteries, located in a compartment at the rear, and
originally had a leather carrying strap. Presumably it was designed
for carrying to an aircraft where it could be used for checking
the calibration of such things as the R1155?
The controls are extremely fine.
That on the left has a built-in counter similar in operation
to the HRO dial except the main ring is fixed and it uses a bakelite
knob with a pointer. This arrangement has a rear indicator which
goes from A to P (missing I) and gives a total of 15 times 360
calibrations or over 5000 individually marked setting points.
Behind the dial is a "roller coaster", a large diameter
coil around which a shorting bar is moved as the dial is turned.
The right hand control is coupled to a large air-spaced tuning
condenser and this is also calibrated to a remarkable degree.
The main dial carries 100 calibration points and the directly
coupled slow motion dial a further 20, 10 of which relate to
a single calibration division of the main dial.
With this sort of accuracy one
needs absolute confidence in the calibration and to this end
a set of charts is clipped to the rear of the box. This gives
inforamtion allowing a setting accuracy of 4 significant figures.
For example the wavemeter was able to be set to a frequency of
5.009Mc/s representing a setting accuracy of 1Kc/s. This sort
of figure means that one cannot touch any of the internal components
or replace valves without, essentially, sending the thing back
to the factory where it may even be required to be equipped with
a new set of charts. The valves carry the serial number of the
equipment as do the charts of course.
In terms of its usefulness in
a modern environment, alas it is no better than an interesting
curiosity!
As a matter of interest. My
data books give equivalents for VR, VT and VU valves but are
silent on the VW36 and VW48. Does anyone know what the commercial
equivalents they are? |
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General
Radio Signal Generator type 605B
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This once fine piece of
large and heavy laboratory equipment was made by General Radio
of Cambridge Massachussets and bears the label "Supplied
by Claude Lyons" of Liverpool, probably around 1937. Its
a Type 605-B. It weighs precisely 72.5 pounds or 33Kgm. Over
the front panel there can be seen are lots of hex-head blanking
plugs which can be unscrewed to access trimming condensers so
that final alignment can be carried out once the equipment has
been assembled. |
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It covers a huge waveband
stretching from 9.5KHz to 50MHz with potentially tremendous accuracy.
Interesting to note the LF feature which corresponds to a wavelength
of more than 31,500 metres. In the early days every inch of spectrum
was needed, as wavelengths below 200metres were thought pretty
useless. At the ELF end of the spectrum there wasn't much room
for modulation. Today such low wavelengths are used to keep in
touch with nuclear submarines.
Its eight frequency bands are:-
9.5KC- 30KC; 30KC- 95KC; 95KC-
300KC; 300KC- 950KC; 0.95MC- 3MC; 3MC- 9.5MC; 9.5MC- 30MC; 30MC-50MC
Output is calibrated, using
direct readings from two dials of 0 to 10 microvolts and a multiplier
of 10/100/1000/10,000
Of course its date precludes
FM and only AM is possible. The VHF band 30-50MC isn't marked
on the wavechange switch and is selected by turning this one
setting after Range G.
Despite its general tatty appearance
all the controls work smoothly. The equipment's appearance has
suffered somewhat as can be seen by the corrosion on its front
aluminium panel. The case is made of wood and this has absorbed
moisture which has caused a chemical reaction with the aluminium,
eating away the panel inner surface periphery and odd points
on the outer surface where the paint has been damaged.
This example has a UK power
supply with an EZ35 rectifier. The RF section uses four American
valves having the UX base popular in the US up to WWII. being
types "76", "84" and "89" plus
a second "76" under an RF cover. Hidden away I also
discovered a 955 acorn triode. Inside the cabinet the condition
is excellent and to make the unit RF leakproof the interior is
lined with copper sheet. Because of its poor external condition
I got it for £1, but I had to drive 50 miles to Hayling
Island to collect it. It may be possible to carry out a decent
restoration because the legends on the panel are engraved and
if the panel is cleaned up and resprayed may be re-filled with
white paint. |
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I've temporarily rubbed
black paint over the front panel to help prevent further corrosion.
Below are some pictures of the
power supply and RF sections removed from the cabinet. |
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Eight screws hold the
power supply in place and you'll find it withdraws sufficiently
to pull out the connecting lead which is terminated in a 4-way
Jones plug. The mains transformer looks strange and that centre
black box is an LF choke. The rectifier is a BVA EZ35 and the
adjacent TCC condenser was made in the UK and the original mains
connector seems to have swapped for a Belling Lee type, |
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Access to the RF unit
requires more than a little determination as it's held in place
by 14 screws and once extracted a further 24 screws hold the
coil pack cover in place. The cover is very heavy and appears
to be nickel-plated copper and measures 1mm thick. Once the cover
has been removed the triode oscillator and RF components are
revealed |
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Under the RF compartment
three valves are visible. On the right a double diode type 84
and behind this a 76 triode. The valve on the left is a type
89, a power pentode. Under the chassis you can see a label "955"
which marks the location of a tiny acorn valve which is used
as a valve voltmeter. |
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Here's the tuning condenser
whose rotor is about 4.5 inches long and has 35 plates suitably
shaped to provide substantially linear tuning scales. The coils
are pretty substantial, wound on ceramic formers with iron dust
cores set by self locking nuts. |
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Most of the valves are
firmly held in place. Even the triode under the RF cover is held
in place by a rubber cup screwed to the underside of the cover
which needs to be forced into position to clamp the valve when
refitting it. |
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