The classic demonstration of the photoelectric effect is discharging a negatively charged electroscope by illuminating zinc or aluminium connected to the electroscope with 254 nm light from a mercury lamp. Their threshold wavelength is ~290 nm. The demonstration with a germicidal lamp is like this: [youtube video][1]

Clothed like a bank robber and wearing glasses, I repeated the demo with copper and iron, but with these metals the electroscope did not discharge. Why are copper and iron, in practice, apparently unsuitable for this demonstration? Their threshold wavelength is ~270 nm, so 254 nm light should induce the photoelectric effect in these metals. Is the relation between the current and $\Delta\lambda = (\lambda-\lambda_{\text{Threshold}})$ very nonlinear?

  [1]: https://www.youtube.com/watch?v=z-3XaXCvjZw#t=0m55s