Abstract
The following corrections should be made to the above paper:
In the first line of the abstract, `Polycrystalline pyrolytic graphite was' should read `Polycrystalline and pyrolytic graphite were'.
In the third line from the bottom of page L637, `in a sample volume' should read `in the sample volume'.
In the first line of page L640, `3 mu m' should read `3 µm'.
In the tenth line from the bottom of page L641, the word `sufficiently' should be replaced with `substantially'.