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For the kids of my electronics tinker club, I thought it would be a great experiment to see how the resistance of a wire is proportional to its length.

Therefore I made a very simple circuit consisting of a single resistance wire with ~1.1m length which can be measured in 10 pieces of ~11cm.

Here's how the wiring diagram looks:

Reistance wire soldering board

(We'll only use an Ohmmeter to measure the wire, so no other parts are needed)

While the circuit is simple enough, I still try all the circuits myself before I give it to the children. And I find that soldering that resistance wire is really difficult.

The resistance wire is labelled RD 100/0.1 and has a resistance of 62.4 Ohm per meter. The brand is Isotan and the fomula seems to be CuNi44. The German data sheet even says "Isotan is simple to process. The alloy can be soldered and welded easily.".

I'm soldering as a hobbyist for more than 20 years and my solder joints are usually not so bad. However, on this circuit, I really sucked. On some solder joints I couldn't even measure the resistance.

Increasing the temperature seemed to help, but the solder joints still look ugly. How does one solder that stuff? And hint on how the kids will manage to solder this?

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  • \$\begingroup\$ I would try to wrap the write around a screw with a nut and a solder tab. The kids can then solder to the tab. \$\endgroup\$
    – Oldfart
    Jan 15, 2019 at 20:49
  • \$\begingroup\$ no conventional soldering, crimp lug will work. CuNi23Mn is better \$\endgroup\$
    – Tony Stewart EE75
    Jan 15, 2019 at 20:53
  • \$\begingroup\$ Did you try to clean the resistance wire surface before soldering using a sharp knife or sand paper immediately before soldering? An alloy of copper and nickel should be solderable. \$\endgroup\$
    – Uwe
    Jan 15, 2019 at 20:59
  • \$\begingroup\$ not with 1% Mn . Datasheet is misleading it is actually CuNi44Mn1 acid flux maybe \$\endgroup\$
    – Tony Stewart EE75
    Jan 15, 2019 at 21:03
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    \$\begingroup\$ REsistance wire here has 2 meanings. . The CuNi44Mn oxidizes too fast so other suppliers say this item has no conventional soldering means. ( ie it resists soldering ) \$\endgroup\$
    – Tony Stewart EE75
    Jan 15, 2019 at 21:38

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Some metals are 'difficult' to solder, they have tenacious oxides that demand a more active flux that you usually get in cored solder.

I bought a short length of solder for aluminium for joining, amongst other things, aluminium. I tend to 'tin' the alli with the aggressive solder, and then use ordinary solder to make the joint to other metals, to economise on my use of the special stuff.

However, it's also good for soldering K-type thermocouple wire, which is nearly impossible with conventional solder, and for some (not all) of my collection of various resistance wires.

Try sourcing some 'alli' solder, it may work for your particular wire. If you want the kids to be able to solder the wire, it will be good for you to tin it first, as the flux fumes are also nastier than those of conventionally fluxed solder.

You might be better off introducing them to 'croc clips', and just leave it with them that some wires are difficult to solder.

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    \$\begingroup\$ You can also get just plain flux and use that on the wire prior to soldering with the ordinary solder. \$\endgroup\$
    – Hearth
    Jan 15, 2019 at 20:55
  • \$\begingroup\$ @Hearth: that sounds like a feasible option. \$\endgroup\$
    – Thomas Weller
    Jan 15, 2019 at 21:03
  • \$\begingroup\$ @ThomasWeller Do be aware that some fluxes are pretty unpleasant chemicals and you should avoid skin contact. Check the label and be careful. \$\endgroup\$
    – Hearth
    Jan 15, 2019 at 21:15
  • \$\begingroup\$ That last paragraph is a wonderful idea, and I'll accept it. Here's what I'll try: we'll not solder anything but use the children to pull the wire in a zig-zag way with ~2m of distance between the children. It should then be possible to measure the resistance and deduce the formula R~L. We can even take the experiment further by repeating it with a 0.3mm resistance wire (not possible on the small board). We'll find out that R ~ 1/A. That's wonderful! \$\endgroup\$
    – Thomas Weller
    Jan 15, 2019 at 21:48
  • \$\begingroup\$ Croc clips allow rapid experiments. If a bright kid spots the instability of contact resistance, then you could guide them Socratically to rediscover Kelvin's 4 terminal method for resistance measurement 'OK, so the contact resistance isn't zero ohms, how could we measure it?' Don't try this with most of the kids, it will go over their heads. Disclosure, I was that bright kid, and it always annoyed me if the teacher wasn't willing to discuss this odd phenomenon I'd just spotted, especially if it invalidated their airy simplification. \$\endgroup\$
    – Neil_UK
    Jan 16, 2019 at 8:12

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