I've been  a northern beekeeper some years *
I'm ashamed to say it took 20 yrs for the light bulb to go on
I do have a semi-reasonable excuse
A 2A extension cord must reach the yard
I hope you can solder and use a meter
The rest of you may find it worth a laugh

The heat source is a "peanut" bulb (from
a tensor lamp?) the size of 2 (shelled) peanuts
12V 50W, this is powered by a small "junk"
computer PS, simalar exist in printers
I run it at 5 or 6V (12.5 or 16W)(6V is 2 in series from 12)
Probably at full power it would overheat & have short life
The bees don't like light so enclose it in metal
The pins on the bulb don't like to tin, dip them hot
into flux several times. Do NOT bend these pins in
the slightest.   ie when insulating be gentle
The leads are (inside)phone wire, all  4 wires used
Being lazy I wondered if offset Al  (printing plates .010" Al) 
would do -- it does, only 1 layer seems OK
Cut and bend an envelope about 3 x 2.5" with tabs
to seal edges.  This will fit between 2 boxes if
you make sure the frames are clean and wiggled a little bit
Wax bits that drop on this do not melt, only soften
It does not seem to injure brood
It probably should be in contact with the bottom 
of the cluster or nearly so
Now the Mk V design is much more interesting
and only a bit more complex
It is a "heat pipe" to dip into a divided division
board feeder (full depth)
Since the pins will electrolyse if into the
feed the bulb has to be in a dry well at the top
of the pipe. Use a all Cu penny, sanded to size,
for the floor of the well & soldered into the .5" Cu pipe
(If you never filled more than 85% full you
could dispense with the well, just glue
the bulb in place with a gob of silicone)
If you cross drill the pipe to get some
convection (warm feed!!!) don't let the bees into the pipe
many would be killed when filling. There should be
at least 1 hole to let the air out

For wintering (a weak lot) you turn it on at 0f or 5f or 
for breeding always at 50f(10c), maybe sooner
It seems to give about 30f(18c) temp rise to the hive
If your are on the ball this will be controlled
by a thermostat (I'm not so together)
These bulbs can be bought for $2
Depending on the winter it would cost $1-2/bulb
for the whole winter. I think this is cost effective
However due to global warming I haven't gone nuts with this,
I've only made 5, 3 of the mk3 & 2 of the mk5 design

* I'm at 95-98 cold percentile.

dave

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