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Where would you find a Crappie? How about an Alewife? Obviously
the answer is North America, in waters at between 65 to 74 and 48 to 72 degrees
Fahrenheit respectively. Yes, of course they’re fish and knowing where they, and
many others, are most likely to be lurking is one of the best ways to catch
them. So what has this weird-looking object got to do with it? It’s all about
fish finding, and the Heathkit Thermo Spotter, Model MI-104 is basically an underwater
thermometer. It has three main elements; there’s the hand-held reel with a
built-in temperature meter. Wound around the reel there’s a long wire, with a
temperature sensor attached to the end. Drop the sensor in the water (attached to a lead weight) and
as it descends (or rises) the meter displays the temperature of the water. A chart on the
side of the case lists 16 fish species alongside the temperature gradients they
are most likely to be found in.
Most of the fish listed are typically found in the US, which is a
good clue as to where the Thermo Spotter was made. Or does it? Oddly enough,
apart from the fish chart, there is no other clue as to where it came from. There are no
markings anywhere on or inside the case. If you want to be really
picky this one could have been assembled just about anywhere. The Heathkit name
is the giveaway and for over 70 years -- with a couple of interruptions --
they’ve been the world’s leading supplier of home-build electronic kits.
The electronic circuitry inside the Thermo Spotter, such as it is,
wouldn’t have taken very long to put together. It comprises the 50uA moving
coil meter on the front panel, a small printed circuit board with four fixed
resistors and two preset potentiometers (for calibration), a momentary action
push-button switch for taking readings, a holder for a single 1.5 volt AA cell
and the sensor module on the end of the wire. This almost certainly contains a
temperature-sensitive resistor or ‘thermistor’ in a sealed brass capsule. You
will have to take that on trust, as opening it up to make sure would be a no-go,
as it would almost certainly destroy it
It was a chance catch on ebay whilst trawling (all fish puns
intended…) through listings for Heathkit test gear. And as so often happens
with oddball items, no one seemed interested in it. The auction was about to
end, and never having come across such a thing before, I was curious enough to
put in a bold bid of 50 pence over the start price of £10. Luckily it was
the only bid, it arrived quickly, and in surprisingly good condition, just
needing a wipe over with a soft cloth and a squirt or two of Flash to get it
looking like new. There was a long expired AA cell in the battery clip.
Thankfully it hadn’t leaked and apart from some gunge on the contacts the
Spotter turned out to be a runner. I know that
through scientific tests (a kitchen sink and an old school
chemical thermometer). After tweaking the twin pots the meter was more or less
spot on. There was just one minor cosmetic issue, though, and that was the
meter’s transparent cover. At some point the original one had been broken or
lost and replaced with one from another, differently shaped meter. It doesn’t
look too bad but I would like to find the correct one. However, it’s an old and
long-obsolete model and the only one I’ve found on ebay so far cost twice as
much as I paid for this Spotter so I’m not holding my breath.
Finding out when it was
made or assembled turned out to be surprisingly difficult Several online
archives have the odd Heathkit catalogue from the late 60s and early 70s, and
some of them include more sophisticated fish finders and related marine kits
but alas, no Thermo Spotters. The mid-sixties date is a guesstimate, therefore,
based on the overall design, components and materials but as you can see
there’s not a lot to go on, so if anyone can help to pin it down, please get in
touch.