Resistivity of amalgams still a mystery

Thats, Keith. I am adding two more newsgroups that should read this.

"Keith P Walsh" wrote in message
...
Readers who have been visiting these newsgroups for some time may
remember my assertions that there doesn't appear to be anyone anywhere
in the world who knows what the electrical resistivity of a typical
dental amalgam is.

Well, my latest Google search on this topic has thrown up a paper
entitled "Resistivity of Silver-Tin Amalgams", by Richard J Schnell
and Ralph W Phillips of the Indiana University School of Dentistry,
which was published in the Journal of Dental Research in 1964.

See:

http://jdr.iadrjournals.org/cgi/reprint/43/4/501.pdf

This paper describes experimental procedures which were carried out in
order to determine the electrical resistivities of a range of dental
amalgams with varying component mixtures, and the results are
presented in a series of graphs.

The trouble is that these results are all wrong.

This is because the values of the resistivities of the amalgams
presented are all quoted in units of "microhms per cubic centimeter".
And this is not a valid unit for electrical resistivity.

The SI unit of electrical resistivity is the ohm metre - check
Wikipedia at:

http://en.wikipedia.org/wiki/Resistivity

Presenting resistivity values in units of "microhms per cubic
centimeter" is nonsensical. It isn't just a question of the relative
size of the unit, nor the spelling of it. Resistivity is not defined
as a quantity of resistance per unit volume - as the units quoted in
the report imply.

In their paper, Schnell and Phillips describe how they determined
their "resistivities" thus:

"From the cross-section area and length of the specimens [of amalgam]
the resistivity was calculated ...."

However they do not give any detail of how this calculation was done.

The correct way to do this calculation would be to multiply the values
of resistance measured for each specimen by its cross-section area,
and then divide by its length - thus giving resistivity in the
appropriate unit of resistance x length (that Wikipedia page is quite
correct on this).

What did Schnell and Phillips do? Surely they didn't divide their
resistances by BOTH the length and the cross-section area of the
specimen in the mistaken belief that resistivity can be calculated as
resistance per unit volume, did they?

Well the units they used suggests that they may have done this. The
only way to find out would be to get hold of their resistance
measurements and work it out (they are quite precise about the
dimensions of the specimen size they most frequently used).

But as it is their results are completely useless and unquotable,
because electrical resistivity is not quantified in "microhms per
cubic centimetre".

Could this explain why these results are never quoted?

In the textbook "Restorative Dental Materials", edited by Robert G
Craig and John M Powers and published by Mosby, I can find values of
electrical resistivity for Human Enamel, Human Dentin, and several
different types of Dental Cements - all given in perfectly appropriate
(though not SI) units of ohm.cm

However, in spite of the fact that they have devoted an entire chapter
of fifty pages to the properties and use of amalgam, there isn't any
indication anywhere as to what the electrical resistivity of a typical
dental amalgam is.

And I am therefore forced to declare that, either in spite of or
perhaps even to some extent because of Schnell and Phillips dubious
efforts, it still appears that there isn't anyone anywhere in the
world who knows what the electrical resistivity of a typical dental
amalgam is.

Keith P walsh

PS, Electrical resistivity is just one of the properties which
determine the electrical behavior of a material when it is subjected
to an applied electromagnetic field. And metal amalgam dental fillings
continue to be placed in children's teeth.