  Many precision measurements of G have produced a spread of results
incompatible with measurement errors. Clearly an unknown source of systematic
errors is at work. It is proposed here that most of the discrepancies derive
from subtle deviations from Hooke's law, caused by avalanches of entangled
dislocations. The idea is supported by deviations from linearity reported by
experimenters measuring G, similar to what observed, on a larger scale, in
low-frequency spring oscillators. Some mitigating experimental apparatus
modifications are suggested.
