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Originally posted by Polymer
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Eric.
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Thank you Polymer. Lots of interesting information from copper.org. Relevant point under physical properties regarding the increase in electrical resistivity with increasing nickel content.
Eric. -
WW2-era German, Austrian coins ? I was curious what Mr.Hocking made of them.
1941 Deutsches Reich 10 pfennig coin reads 27.2 according to Mr Hocking. A reading of 27 is correct for die cast zinc. A larger size 1 franc French coin dated 1943 reads 30.0, but is of much lighter weight and must be an aluminium alloy.
Eric.Comment
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Ordered a gram of 8mesh nuggets. Hoping signal will be high enough to chart TC decay curve. Any guesses what the TC will be?Comment
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Thanks for testing the zinc coin, Eric, nice that you've confirmed what the numismatists say. And yes, French coins would be aluminium at that time, I've got a couple of them amongst my collection, very light.
I've got my 11 x 7 search-coil bodged into life, and have been trying out the dynamic and static target measurements. Needless to say, nothing quite agrees, so I've got to look into that if I'm going to get good data. It was never intended to be a super-precision instrument, and measuring target frequency to +/-5% was plenty good enough. Now I'm trying to get precise, I'm seeing discrepancies.
Some tests I have done include working out the ratio of "circular target" : "square target" Time-Constant, as these aren't dependant on absolute calibration. I'm using a series of thick foil targets from 10mm to 40mm in both profiles. Plot two straight-line graphs, the slope ratios give the TC ratio. I'll post the details on another thread, but I'm pretty confident that:
circular_TC = 0.94 x square_TC
Some measurements don't seem that good, notably those of the core of a 'zinc' US 1 cent.
I've made a set of 20mm copper rings , when I've worked out what I'm doing, I'll post on the 'target tester' thread.
Green: Can you please explain what "8mesh" is ?Comment
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Not sure but think, www.espimetals.com/.../technical-data/334-understanding-mesh-sizesOriginally posted by Skippy View Post
Site I ordered from suggests 7 to 12 nuggets/gram.Comment
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Not sure but think, www.espimetals.com/.../technical-data/334-understanding-mesh-sizesOriginally posted by Skippy View Post
Site I ordered from suggests 7 to 12 nuggets/gram.
I'm still working on(TC=D*T*%IACS*multiplier). Multiplier doesn't seem to be the same for all targets. Maybe materials, target size, %IACS or other? I'll post what I find on 'target tester' thread.Comment
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I've corrected the duff link:
http://www.espimetals.com/index.php/...20Mesh%20Sizes
So 8 mesh is basically 8 holes per linear inch, with the wire size making them smaller than 1/8th inch.Comment
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This paper might be of interest, particularly those with a flair for maths. The reason is that it deals with rectangular cross sections; obviously including square, and in the appendix, a thin plate where TC is independant of area at later times for a constant thickness. I have the rest of the paper which can be downloaded from Researchgate for those interested. You have to join though.
Eric.
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Thanks. Not what I get if I understand the statement correctly. I get TC proportional to width and thickness, thin or thick plate. Maybe do to difference in test procedure or area is change in length not width? Wish I understood the math.Originally posted by Ferric Toes View PostComment
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After looking at the paper again I'm wondering if my experiment where TC of a piece of copper wire doesn't change with change in wire length isn't similar to what they are doing.
Looking at there chart (figure2) for .94cm copper rod. About 600usec/decade(TC=261usec). I calculate 265usec using the data from my wire experiments. The same as theirs since the 600usec was a best guess.Comment
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Wire(TCusec=Dmm^2*%IACS*.03)from my test data with copper wire.Originally posted by green View PostComment
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Generally b>>a would mean that b is probably several orders of magnitude larger than a. However, b and a are the sides of a long bar. I would have thought that a 'c' would have been introduced as a thin slice of the bar i.e. if a = b for a square bar then a=b>>c. Otherwise we have a thin strip parallel to the solenoid field, which is not very helpful. The comment at the bottom of the page was made by a Graduate in physics. Perhaps Skippy can help us out.Originally posted by green View Post
Eric.Comment
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Received 8mesh nuggets. For a quick test I charted all the nuggets at same time(side by side not touching)to see how much signal. Decay TC 1.7usec, good signal, think I can test them one at a time flat. TC on edge might be to low. Anyone familiar with small nuggets, do they look like what you would expect?Comment
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That are exactly what I would expect small nuggets look like. Compare with my picture in post 247. Some of mine (bottom right and some in the pot) are more rounded and dull due to abrasion in a stream bed. The three at the top are undetectable, except on a Fisher Goldbug 2, as they are just a bundle of filaments.Originally posted by green View Post
EricComment
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"Perhaps Skippy can help us out"
Hmmm .... that solenoid test rig measures the bar edge-on. So you need to compare their results with a metal detector seeing the target edge-on. For the thin plate case, the circulating eddy currents are just going round in tiny loops the diameter of which equals the plate thickness, a. There's lots of the loops, due to the long b dimension, but that just affects the amplitude of the response. In their formula, you can see the amplitude includes the ( b/a ) term , but the time-constant in the exponential decay just includes a . The 'long' plate in their set-up becomes a very 'thick' plate to us, and once it's more than '5a' thick, it makes little difference.
So this does compare sort-of with our "straight bits of wire" tests. We don't test them end-on, we test flat. So the wire diameter approximates the 'a' , the length of wire is the 'b' , and we fail in the thickness, by only having one wire-diameter, not lots of metal.
However, I've struggled with a couple of things in that text. The decay time-constant formulae don't seem to produce the correct values, by a factor of 100+, and the figures for a round bar and a rectangular one don't 'match up'. For the round bar, they specify its radius, R . For the rectangle, they specify the linear edge dimensions, a, b. They seem to say a 10mm diameter, 5mm radius round bar behaves like a square one 7.1mm x 7.1mm. Must have another read.
Gotta say, Greens nuggets look too big to fall through a Number 8 mesh.Comment
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