Even more laboratory test equipment

Resistance box

 This looks slightly military but could be school physics lab! You're looking at the outer box which contains the resistance box inside. Total resistance is 110ohms and one of it's uses could have been in an arm of a Wheatstone Bridge when it would have been used for measuring the resistance of a circuit. Particular resistance coils may be switched into circuit by removing a tapered shorting peg and are so arranged that any value from zero to 110ohms is possible.

A typical military use would have been to measure the resistance of a circuit for detonating an explosive charge. Similar boxes were also used for measuring the resistance of telephone lines but these had a much higher maximum resistance, of the order of 10,000ohms. The method of using the box is to construct a Wheatstone Bridge carrying four arms. One is the unknown resistance and two are fixed. The fourth is the resistance box. A sensitive, centre-zero, meter is connected across the middle of the bridge, and by adding or removing the pegs, the meter reading can be made to vary. When the meter reads zero current the unknown arm is equal to the resistance calculated by adding up the coils whose pegs have been removed.

Although instruments like this have been around for more than 100 years, this example appears to date from the turn of the century. The previous owner thought the box may be as late as the 1950s but on the base of the inner box, in tiny inked writing, is the date 2/2/05... so it's Edwardian.

On the front of the outer box can be seen a metal label. This reads "No.R5 OHMS 110". However, did the older screw holes (just visible in the picture) around the plate, originally hold a military label?

 On the inner box ebonite top is inscribed the maker's name "Robt.W.Paul, London,N". The outer box is made from what to me looks similar to pine and the inner in mahogany. Joints of both boxes are not dovetailed but are of "butt" form. The outer box joints are nailed and those of the inner, which is beautifully made, are secured by brass screws, whose heads are perfectly flush with the wood.

Comments are invited from resistance box collectors!

Switch and Rheostat

 Looks like a survivor from WWI, but I've not really any idea what it is. If anyone knows what it is please let me know!

Well when it arrived it was obvious that what you see is only a part of the original. The brown ebonite front panel has been cut and the box on which it has been fitted also cut down to fit.

I suspect it may have been a WWI receiver front panel. The rheostat may have been used to vary a valves heater current (an early volume control). This however is a guess and I await to be corrected.

Inside the very tatty case....

a wirewound rheostat in perfect condition with intricate set of pulleys and steel wire.


Cossor Wavemeter/Grid-dip meter from the 1950s

Click the picture to see more about this

 This extremely useful tester was used to check the resonance of a coil and capacitor combination.

The appropriate coil was plugged in and this was then held adjacent to the tuned circuit under test. The grid dip tuning was then changed until a sharp dip was obtained on the meter. This indicated resonance. The principle was quite simple. The grid dip meter contains a wide band oscillator. When this was set to the the same frequency as the natural resonant frequency of the tuned circuit under test, power would be absorbed by the latter, reducing current in the oscillator circuit, resulting in a much reduced meter reading.

The device could also be used to determine the inductance of a coil. A known capacitor soldered across the coil would set a resonant frequency given by f=1/2*pi*root LC.

One could also use it as a simple signal generator or for testing aerials.

Philco Radio Testmeter

 A Model 048A tester made by the US firm of Philco.

It uses a single UX based valve and includes a signal generator as well as having numerous measurement facities. In fact it is supposed to include everything needed too service a pre-war radio.

Measurements, courtesy of Marconi

 On the left an RF power meter and on the right an audio power meter.

These are both ex-military equipments made by Marconi

Quite useful things... one I use for testing transmitters and the other, in conjunction with a signal generator, for aligning radios.

Larger picture of the absorption wattmeter A.F.N. No1

Yet more (is there no end to this stuff!)

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