From J. W. Strutt to G. H. Darwin [19 June 1879]

My dear Darwin

I believe pure copper (electrolysed) can be had, but perhaps even then diamagnetism might interfere. You could take two precautions against it, (1) by working in the position that diamagnetism alone w^d. tend to give, (2) by measuring the current w^h produces the field (easily done), & having it always the same. But perhaps for such great delicacy this might not be enough. However I should fear still greater disturbances from a liquid as you proposed. I am not clear that you ought to have any damping at all. If the oscillations are slow enough, the extreme excursions can be read, & contain all the information there is to be had. If you use damping, preliminary exp^ts will be necessary to try different ways. There would be no harm in using a copper bob anyhow, & ther is no need to turn on the Electricity, even if you can do so. I have had no experience of Electric damping, & so my views are not worth much.

Yours very truly | Rayleigh