Compass

A section dedicated to non Mini related conversations that some members may find disturbing, worrying or offensive. If you are likely to be troubled by non Classic car related content, I would suggest you avoid!
Cammsjb
Posts: 346
Joined: Sat Sep 12, 2015 7:40 pm
Location: Lancashire

Compass

Post by Cammsjb »

I found this compass in a box of old Mini parts today, has anyone seen one like this before?
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surfblue63
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Location: NE England

Re: Compass

Post by surfblue63 »

My grandad had a similar one in his Capri and I had a mate a college with a Rover 2000 TC with one. Analogue Sat Nav, in it.
Cammsjb
Posts: 346
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Location: Lancashire

Re: Compass

Post by Cammsjb »

Anologue sat-nav I like that.

It looks like a 60s-70s gadget, maybe quite rare now, the clear plastic half sphere is filled with a clear liquid, not sure what it is, the direction indicator sits on a pivot pin and turns slowly when the direction changes, sticks on a windscreen or dash.
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surfblue63
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Re: Compass

Post by surfblue63 »

You can still buy them now. The liquid is probably some sort of oil to dampen the movement, otherwise it would just vibrate itself to bits.
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Peter Laidler
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Re: Compass

Post by Peter Laidler »

I'm left wondering how a compass, even a damped one, can have any sort of accuracy when it is contained within a steel box such as a car. I appreciate that it looks like it's stuck to a windscreen but there's still an awful lot of steel surrounding it. Maybe it's me. Not sure I'd be happy flying a bomber to Dusseldorf tonight using one!
GraemeC
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Re: Compass

Post by GraemeC »

I very much doubt they expected anyone to do that with their automotive aftermarket trinket
hanlminiman
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Re: Compass

Post by hanlminiman »

Peter Laidler wrote: Sat Sep 25, 2021 2:35 pm I'm left wondering how a compass, even a damped one, can have any sort of accuracy when it is contained within a steel box such as a car. I appreciate that it looks like it's stuck to a windscreen but there's still an awful lot of steel surrounding it. Maybe it's me. Not sure I'd be happy flying a bomber to Dusseldorf tonight using one!
Peter. If I remember correctly there was a screw on the unit to get the setting correct. Perhaps that's why I ended up with a "useDfordle" and not Dusseldorf (happy memories from my teenage years touring the UK military bases!)
:D
Cammsjb
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Location: Lancashire

Re: Compass

Post by Cammsjb »

Thanks for your comments.
I've done a little more digging relating to this compass, I have now found out that it belonged to my father-in-law who had been in possession of it since the late fifties, at the time he was working in the aircraft assembly industry and possibly came from there.
There isn't any adjustment function on the compass or manufacture's mark but the directional operation of it is spot on when compared to a car sat-nav.
Dearg1275
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Re: Compass

Post by Dearg1275 »

Yes the needle of the compass will be attracted to close proximity to the metal of the car’s shell but that attraction, given the weakness of the magnetism in of the needle, is weak. So little deflection if mounted on a windscreen, typically centrally. Also, the shell of the car will also be affected by the the earths magnetic field. This would tend to align the field within the shell with that out side owing to the earths field. The magnetic north in the car would be at all times broadly align to terrestrial magnetic north.

If this is all a load of cod’s based on poor knowledge of physics please let someone come up with the definitive answer. Faraday cage theory only applies to static electricity and not to magnetism.

D
1071 S
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Re: Compass

Post by 1071 S »

This is an automotive doodad which was a fad (and actually quite useful) in the days before GPS. It was good to know which way to hold the map (north is traditionally up ....but which way is north??).

Any errors are not connected to a Faraday cage but the effect of a large lump of metal (ie car body) on the local direction of the earth’s magnetic field.

As the vehicle body turns through north, east ...etc.. the effect on the earth’s field (that the compass needle is aligning to) varies. Wonder of wonders, this error is officially known as “variation”. If you’re feeling really serious then it helps to know that “Declination” is the error between the magnetic field (magnetic north) and the direction of “true” north - ie the top end of the axis around which the earth rotates....The error angle and rate of change (true north is pretty much fixed while magnetic north sort of floats around) is printed on all serious maps.

Once upon a time, Her Majesty used to pay me to “swing the compass”. We would literally drag the ship in a circle around its anchor and measure the variation error as the ship’s head adopted different directions when compared to (local magnetic) north. Variation would be minimised by adjusting the Flinders Balls (named after a young Pom who is a particular hero of mine).

The combined variation and declination errors only total a few degrees so are not a serious concern for a device only intended to (roughly) indicate the vehicles orientation.

Luckily its Sunday morning so, if anyone does read this treatise, then there hasn’t been any real waste of time.

Cheers, Ian
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