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!


 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?

 Signal Generator from just before WWII

 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 1936 or 7. Its a Type 605-B.

 

 It covers a huge waveband stretching from 9.5KHz to 30MHz 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 wavelengths are used to keep in touch with nuclear submarines.

Its seven frequency bands are:-

9.5KC- 30KC; 30KC- 95KC; 95KC- 300KC; 300KC- 950KC; 0.95MC- 3MC; 3MC- 9.5MC; 9.5MC- 30MC

Output is calibrated, using direct readings from two dials of 0 to 10 uVolts with a multiplier of 10/100/1000/10,000

Of course its date precludes FM and only AM is possible

Despite its general tatty appearance all the controls work smoothly. Unfortunately the equipment has suffered somewhat as can be seen by the corrosion on its 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 and uses four American valves having the UX base popular in the US up to WWII. The three in the RF unit are types "76", "74" and "89". Inside, the condition is excellent and to make the unit RF leakproof, the interior is lined with copper sheet. Because of its condition I got it for my first bid of £1. It may be possible to carry out restoration as 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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