And even more Laboratory Test Equipment!

 Foster Portable Potentiometer Model 3155

 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.



 A metal-cased scientific instrument donated by my friend Mike


 Cambridge Instruments Potentiometer

  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.

 General Radio Wavemeter, Type 558-P

 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.



 Below: The wavemeter with one of the five coils fitted




 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


Click here to see Bulletin 931 mentioned in the above advert

Page 14 has the Type 558 Wavemeter

  This is the W1117 a WWII wavemeter made for the RAF

 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.


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? 

 General Radio Signal Generator type 605B

 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.


 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.


 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.



 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,



 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


 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.



 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.




 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.


 Click to see the operating instructions for the earlier Model 605A

 See an article from the General Radio Technical Bulletin from November 1937


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