GPO Cathedral Galvanommeter

[markgengine]

Hi all.
Just aquired a long sought after GPO "Cathedral" galvanometer in used (& abused) but functioning condition. It was missing a couple of brass 2 BA csk screws securing the mechanism to the wooden case, luckily I had these. I'd not had one of these apart before and it was immediately apparent that the outer cover does not simply slide off, secured by the top rear screw, but needs to fit over the two steel pins arrowed in the pictures.
These solid but seem as if they will move given a tap, but am wary of blindly diving in and looking for any knowledge from group.
I wonder if this unit has been part of another larger instrument or is just lacking a base board ? that would have retained the pins if they were a loose fit. The pins would have to come out for the top cover to slide forward, and it seems strange that these pins do not have any obvious way of gripping to remove them, have they been cut down or otherwise modded ?
I'll be making a base anyway as I am also concerned about the hole in the base that exposes the movement magnet so any loose bit of iron about will get sucked up into the movement. The case has two mirror plates on the base which appear original.
Only markings are the "G.P.O." on the dial, "30" stamped on the main case back and "24" on the hidden part of the top case holding the glass.
Otherwise I just have a familiar quandry about how far to go with restoring the outer case, the term "patina" is pushed a bit toward the dirty side so I am tempted to re French polish it but in a sympathetic way.
All advice welcome.

[Diabolical Artificer]

Blimey, they went mad on the case, is that mahogany? It the case carved out of a solid chunk? No wonder they're nearly extinct. Nice device, perhaps just give it a light clean & brush any dust off & give the case a bit of Danish oil thinned down.

Andy.

[markgengine]

Yes the outer case with the glass is made from one solid piece of mahogany, the "static" part is of two pieces jointed and screwed, no sign of any glue. Searching t'net shows very few examples and the mystery of the steel pins remains.

[pmmunro]

Mark,

Could these be steel locating pins, simply pressed in? If so it might be worth heating them with a soldering iron to see if that loosens them. You could also try making a jig to press them out; a piece of flat bar drilled and tapped clamped to the board would possibly be suitable.

Do you know the date of the galvanometer? The green cotton(?) covered wire would suggest pre-1920 as it was found around 1912 that the then-available insulating varnishes alone gave sufficiently good insulation.

PMM

[markgengine]

Sorted !
Good example of a piece of gear that has "been got at"
On each side of the movement there is approximately 2 foot of green silk covered wire, this should be neatly coiled according to other pics I have seen. As I'd mentioned before, the two 2 BA screws securing the movement were missing as found.
I assume that some rascal removed all the screws on the back and the heavy movement fell out and was left hanging on these wires with a resulting hideous rats nest. (see pics)
The resistance accross the movement and connecting wires is 30 ohms (30 is stamped on the back) so these green wires serve as a series resistance to the movement.
The resulting tangle of wire was then stuffed back in the case and the cover jammed on by aforementioned rascal. That is why I could not open the case easily as the wire jumble jammed the case from lifting out over the pins, and led to my bafflement by the base locating pins.
I have very gingerly re-coiled these wires that are some stiffish resistance alloy. I had one snap and to rejoin I burnt off a small ammount of insulation, the smell identifies it as definitely silk not cotton. after careful scraping, has soldered OK.
So now the case easily opens by removing one screw and livting up slightly before pulling back.
Resistance is 30 ohms accross the rear terminals and FSD to number 60 on the un-united dial is 250 mA
The case is cleaning up well with beeswax polish. I still wonder about the open hole in the base exposing the coil magnet, seems odd with such a carefully made piece of mahogany to leave such a big hole for debris to enter.
So just a base to make and then try and find the end of that trans-Atlantic cable !

[markgengine]

Oops Typo on above 25 mA & Not 250 FSD

[Silicon]

Are there any ventilation holes to allow the cavity to 'breathe'?

Maybe the large hole was originally plugged with a wad of thick felt to keep the dust out.

[Gridiron]

I have one of these, virtually identical but mine does not have the fixing lugs on the base. I think these galvanometers were intended for mounting on a telegraphy baseboard so the hole in the base would be covered by this.
The book "Telegraphy" by Herbert has a description. The movement used is the Varley pattern with the two vertical magnets mounted so that their magnetism is reinforced by the Earth's vertical field magnetism.
The book I have is from 1920, by then it states the Varley movement was obsolete, the Spagnoletti movement was the one being used.
The "Practical Telephone Handbook" (Poole)of 1905 has a photo of one of these instruments mounted on a telegraphy baseboard so these probably date back to the Victorian period.
Mike.

[markgengine]

Thanks Mike, I have seen a few images of these mounted on baseboards as you say, I also note that in the few images on the net of the underside you can spot "whiskers" of iron particles.
I would have expected, as Silicon suggests, that there would be at least a felt cover, but I see no evidence. A piece of masking tape is serving as a tempary cover.
I would be interested in exactly how these were used in practice, in railway telegraphy the needle is just either left or right of zero, why graduate the scale in (apparently) arbitary numbers ?
Also, this is yet another very well and expensively made piece of early 20th (?) century equipment I have that strangely has no makers name marked on it

[Graham G3ZVT]

I wouldn't call the scale arbitrary, it depicts +/- 70 degrees of rotation accurately.

[markgengine]

Yes but a scale where the units are not indicated on the instrument are meaningless, possible angular measurement is of no meaning electrically unless there is comparison with an identical instrument, bit iffy even then.

[Gridiron]

Looking again at the telegraphy book I mentioned in my previous reply, this type of galvo with a scale was introduced for use with central battery telegraphy systems primarily to indicate any leakage on the line, the diagram shown has 4 uF capacitors at each station which charge and discharge to operate the sounders.
Mike.

[markgengine]

Thanks for the additional info Mike, any chance of a scan or photo of any relavant diagrammes etc ? After wanting one of these instruments for a while I am now wondering how they were actually employed.
Mark

[Gridiron]

Attached a scan of the relevant pages in the book, hope it is readable.
Mike.

[markgengine]

That's excellent Mike, thanks.