Page talk:A Treatise on Electricity and Magnetism - Volume 1.djvu/69

See the letter in red, I can'r read it --Alex brollo 13:42, 4 July 2008 (UTC)

The formula looks like $q_{0}-{\bar {q}}={\frac {1}{10}}r^{2}\nabla ^{2}q$ to me. The factor of one tenth looks strange, but stuff like this can happen. The explanation can probably found in the cited location. Or we try work it out ourselves. I'm too lazy to do that now but I think what will happen is this: since $r$ is infinitesimal, it is legitimate to write ${\bar {q}}$ as a spherical integral, with the integrand expanded only to second order in $r$. Zeroth order should give $q_{0}$, first order should fall victim to spherical symmetry and the $(i,j)$ and $(j,i)$ derivatives of second order should kill each other whenever $i\neq j$, so that only a Laplacian remains. It should then be easy for any diligent student who is not scared off by the dripping sarcasm in this sentence to work out the factor ;)--GrafZahl (talk) 09:36, 8 July 2008 (UTC)

I'm crap with complex math like this, but it does at least look like a "0" (zero) to me. EVula // talk // 01:00, 18 September 2008 (UTC)

When magnified it is clearly seen that this is not 1/10 but some strange designation with 1 at the top and two separate dashes above 1 and 0 in the bottom ${\begin{aligned}&\ 1\\&{\bar {1}}\,{\bar {0}}\end{aligned}}$. I do not know what this designation could mean at Maxwell's time. Maybe it is an order of smallness. --Astrohist (talk) 12:28, 9 June 2010 (UTC).