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Robert Eno asks about a Tritium-illuminated handheld compass.

Tritium is a radioactive gas, an isotope of Hydrogen, so chemically it
behaves just like Hydrogen. It is sealed into a tiny glass capsule, which
has a green phosphor coating inside. The low-energy beta-particles, that it
gives off as it decays, excite the phosphor into giving off light. It has a
half-life of a bit over 12 years, which implies that after 12 years the
light output is down to half its initial value, after another 12 years it's
down to a quarter, and so on, as its initial stored energy gradually runs
down.

On the other hand, because each disintegration produces so little energy, a
lot of disintegrations per second (measured in Curies) are required in the
light-source, to provide sufficient light. This figure tends to frighten
the uninformed.

The beta-ray particles that Tritium emits have such low energy that they
don't exit the glass capsule. Even if the Tritium gas were to be released,
these betas wouldn't pass through your skin, or even a piece of paper.
Hydrogen, as a gas, isn't readily absorbed in the body; if you breathe it
in you will breathe it out again. If it should be burned (by passing
through the flame of an oil cabin lamp, say) it would become Tritiated
water-vapour, which like normal water-vapour is readily absorbed. That
would increase the hazard greatly.

As Peter Smith rightly said- "Tritium is
pretty benign, being a low-energy beta emitter, but if you managed to break
the capsule and swallow the stuff... well, I leave that to your lawyer's
imagination."

There was a time when literally millions of phones in the UK were fitted
with Tritium illumination of the dial, which was not regarded as any sort
of hazard. In the end, when these became obsolete, they were gathered
together for scrapping in enormous numbers. And that gave rise to a great
disposal problem.

Which brings me to a situation where Tritium lighting can conceivably
become dangerous, There was a time (and it may indeed still be the case)
when civil passenger aircraft had to be fitted with emergency lighting to
indicate exits and escape routes which would keep going even if all power
failed, and Tritium lighting, in large quantities (far, far greater than
any compass requires) was installed to provide it. In the event of an
aircraft crash, you would come down sealed into a cabin with lots of
Tritium-containing glassware, and perhaps with a fire to convert it into
hazardous Tritiated water-vapour. Well, perhaps in those circumstanes you
have more pressing matters to worry about...

Another point about Tritium is that it's a component of the Hydrogen bomb,
but you would need an awful lot of light-capsules, and a lot of other
technology, to make anything of it. It's just another factor which excites
a degree of official nervousness whenever the word Tritium crops up.

===================

Getting back to compasses, I wonder if the solid aluminium body of Robert
Eno's Sisteco serves a special purpose. If you surround a moving magnet
with thick high-conductivity material (and Aluminium is second only to
Copper in that regard) the eddy-currents that are induced in the metal will
act to oppose any movements of the magnet, and so may provide a mechanism
for damping of compass oscillations without need for any liquid damping.
Does Robert Eno's compass manage to do without any liquid damping, I
wonder?

For years I have used liquid-damped compasses of the hockey-puck type,
originally designed by the French firm Morin, sold under various labels as
the "Opti Compas" or "Mini Compass". These are very simple, clever, and
effective. A 10-degree wide patch of the scale engraved on the rotating
transparent card appears, magnified, in a prism, as you look over the top
of the puck. At night, it's lit from below, either by a Tritium light, or
you can now buy a fluorescent version.

Here comes the really clever bit. That 10-degree-wide scale that you can
see in the prism coincides exacty with a 10-degree-wide patch of the
horizon that you see above it. Below any object on the horizon, its
corresponding bearing is shown on the scale. You can twist the compass
around a bit, or move your head,and provided the object stays within that
10-degree arc, its correct bearing is shown below it. As a result, there's
NO NEED FOR ANY PRECISE ALIGNMENT OF THE COMPASS! So it has no sighting
vanes, because none are needed. It may be (I don't know) that other makers
have cottoned on now to this simple trick, but Morin was certainly the
first to get the optics right in that way.

The only complaint I have had about the Mini-compass is the gradual dimming
of the Tritium illumination, which has combined with a similar dimming of
my old eyes to make it nearly useless in the dark, after many years. In the
UK, although some traders will sell me a new compass fitted with a Tritium
light, I have found none who will supply or fit a replacement Tritium light
to an existing compass.

I have found that the light-capsule is held in place with an aluminium
spacer, by a rubber disc which acts as a plug. One owner has told me that
in his compass the capsule had at some time fallen out and vanished: in his
case, then, disposing of it was not a problem. It's possible to wheedle out
that plug with care and remove the capsule (which is fragile). This is a
job you would do at your own risk and I can't advise about any regulations
that might apply in your country, nor about how to dispose of the capsule.

You can use a LED (light-emitting diode) instead as a light-source. From
memory, the LED I fitted was rectangular, something like 5mm by 2.5 mm,
which fitted nicely into the spacer and illuminated just the right area of
the compass-card. I have found a red LED to work well. You don't need a
lens-type; just one with a flat face. You need only a milliamp or less of
current, so battery drain is no problem. As a power source, I've used a
3-volt calculator battery, a resistor of a couple of thousand ohms or so to
limit the current, and a push-button switch. You might consider fitting two
such push-buttons, with differing resistors, to give a choice of
illumination to match the ambient brightness. A good subject for tinkering.

One difficulty is finding a suitable coin-cell that doesn't contain any
ferrous material and so doesn't affect the magnetic field. I was unable to
find such a battery so was driven to dangling that little calculator-cell
on its wires, 6 inches or so below the compass, which was far enough to
remove its influence. Not a very satisfactory solution, and if anyone knows
of a suitable non-magnetic coin-cell I would be pleased to hear.

George.

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contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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