An interesting little engineering device

Originally published at:


Apparently this thing is called a Roebling device

I seriously doubt that. It looks like a general-purpose device that’s been branded, perhaps as a promotional item (like a pen with your business name on it).

Look in the middle: It’s made by Novelty Electric Co, not Roebling.

ETA: Called it.


Neat tool, but it really only measures solid wire gauge and displays (rather than calculates) the corresponding resistance per foot. That is more convenient than having to look up the values in a book of tables.


This particular gauge might have been a promotional gift handed out by John A. Roebling’s Sons Company. They made wires.
(As in John A. ‘designed the Brooklyn Bridge’ Roebling.)


Wouldn’t the resistance vary depending on the composition of the metal being used for the wire? Seems to assume that all wires are made the same. Probably not that big of a deal but i am curious, this is not something i would be knowledgeable about.


Probably meant to use with the “standard” copper wires of its time.
Or the wires made by that particular brand.


yeah this only works for copper. any wire made with metal of a different resistivity wouldn’t work


The BB post only says “wire” which to me didn’t indicate what kind it might be hence my confusion but copper does make sense :slight_smile:


Besides, how much wire isn’t copper?

Go back far enough, and it would be all copper. Aluminum wire was used for some time for house wiring, but after the fact it’s deemed a bad choice.

I don’t know where else aluminium wire was used. And while I’ve seen tinned copper wire (and both solid and stranded wire), I can’t think of seeing other types, at least for wiring.

So expecting copper wire seems reasonable for this gadget.


I think I recall hearing that for certain high-tension transmission lines aluminum is used due it both being more lightweight, and being able to be formed as a tube (both weight advantages and for skin effect reasons).


For us AWG types, there’s a useful and simple relationship by remembering one number and your log approximations.

Start with AWG 10, which is 1 ohm per 1000 feet. Then any other wire resistance is based on log10(), so a factor of 2 is 3 steps (i.e., 10*log10(2)), 2.5 is 4 steps, 2 steps the sqrt of that or about 1.6, and a full 10 steps is a factor of 10.

So AWG 20 will be 10 ohms/1000 ft, AWG 22 is 16, and AWG 24 is 25.

It’s not quite exact but often good enough for back of the envelope.


Aluminum has 60% of the conductivity of copper but 30% the weight. It’s also a lot more common, and thus cheaper than copper by the pound, in many cases making it a superior material on the face of it. BUT there are two reasons it’s not used any more in residential applications:

  1. Aluminum wire expands and contracts a LOT more than copper with temperature changes. Many house fires were caused by switch boxes, junctions, splices, and similar connections loosening and eventually failing, causing shorts and fires.

  2. Galvanic corrosion is a lesser but still common problem, wherever aluminum and copper meet, especially with current flow. Again, this can cause hot spots, shorts, and fires.

Back in the '80s, you heard about the 2nd reason more often; not sure why it’s mostly ignored, now, it IS a real effect.


The resistivity of conductors depends on the temperature of the material. Also, BTW, the AC resistance of a conductor depends on the frequency of current flowing through it (skin effect) and for layered conductors, proximity effect further increases AC resistance as a function of frequency and number of layers. So ideally such a device or lookup table would at least qualify the temperature at which the values are valid.


Copper is the best choice for electrical wire almost always. Cheaper than gold and conducts better than aluminum. That’s not to say that that aluminum isn’t pretty common in the real world. It’s gets used in home wiring in houses of a certain era, and it gets used a bit for rural utility lines.

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IIRC, in the British post-war telephone network. Then a cost-saver, now bad for bandwith.

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Is that a copy protection code wheel?


I spent a good portion of a summer working in the cable yard of a utility company doing inventory, and a lot of the high voltage stuff was aluminum.

Funny, I never saw one of those devices, though.


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Various alloys (though AFAIK they’re all mostly copper) are used for high-speed train catenary. You need a certain amount of tension in the wire so that the wave produced by the pantograph doesn’t get caught by the train- the tension required for high speed rail means that plain copper isn’t strong enough.


…and excessive expansion due to temperature variation is BAD. My recollection is that the lower wire is often actually two wires, with the bottom wire clipped to the upper one so that as it wears it can be more easily replaced.