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Hands-On-Physics
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Core
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- Soldering - |
Molten solder won't wet dirty, corroded or oxidized surfaces. Some metals,
aluminum in particular, always have an oxide surface and won't solder. To
dissolve that oxide and to prevent the formation of more, rosin is added
to the tin-lead alloy of the solder--the same rosin obtained from pine trees
and used on violin bows--plus " activators." This flux smokes
when hot, but the smoke is not harmful in reasonable amounts. Tests also
show that the hazard of lead vapor from soldering at this scale is negligible.
A new soldering iron should be tinned--covered with a shiny film of solder--as
it heats up for the first time, to prevent corrosion. Cover the entire tip
surface with fresh molten solder, then wipe it quickly with a damp sponge.
The solder should flow onto the tip readily. If the tip gets pitted and
corroded after some use, it will appear black. Soldering will become difficult:
there will be no liquid surface to conduct heat efficiently from the iron
to what you are soldering. To improve matters, cool the iron and try filing
or sanding the tip clean, then reheat and tin with fresh solder before the
tip re oxidizes. This may fail on plated steel tips, but tips can easily
be replaced. For permanent work it is usual to start by twisting wires around
each other or around lugs--metal tabs with holes--to make a mechanically
strong joint before soldering. But this makes it hard to change your mind
or make corrections.
Figure S3
Solder joints.
Prepare clip leads
Make at least a couple of clip leads to use as easy temporary connectors
in electrical assemblies. They are made from a wire with two little clamping
devices--alligators!--on the ends. (You might also use ready-made clip leads.)
We like to use multi-stranded wire for these, since it is more pliable than
the single-stranded alternative. All people who work with things electrical
recognize a simple color code for leads: black stands for ground or negative
and red for hot, positive. Make yourself one clip lead of each color and
you will come to see how useful this little convention is. (A different
color convention governs household wiring, alas!) To attach an alligator
clip the end of a wire, use your wire strippers to expose about a half-inch
of the multi-stranded, uninsulated wire. The stripper has an adjusting screw
to control the diameter of the hole in its jaws. The size of that hole should
be adjusted to cut and push away the insulation, uncovering the bundle of
wires. Check the piece you cut off: if some of the wire strands got cut
away, open your stripper jaws a little and try again. One way to get it
right is to try the stripper on the stripped wire and adjust it for no drag
as you move it across the wire. Twist the exposed wire so it stays together
in a bundle instead of fanning out. It's a good idea to tin the twisted
wire bundle. Having stripped your wire ends, slip the color-matching insulating
sleeves, small end first, over the wire. Push them out of the way. Now slip
the plastic insulation of the wire through the metal tabs at the end of
the alligator. The bare wire should go through the hole and bend down to
contact the metal of the alligator; clamp the metal sleeves of the alligator
onto the insulated wire with pliers. Solder the end of the wire to the clip.
(Some alligators have a clamping screw; if you use those you can solder
or not as you choose.) You'll need an alligator at each end of the lead.
Pull the sleeves over the alligators. You can work the jaws of these beasts
through the covers. (In our case, without dangerous voltages, the covers
act more to prevent the metal parts from touching than as safety shields.)