  Models for same-material contact electrification in granular media often rely
on a local charge-driving parameter whose spatial variations lead to a
stochastic origin for charge exchange. Measuring the charge transfer from
individual granular spheres after contacts with substrates of the same
material, we find that the charge-driving parameter is global, not local.
Cleaning and baking samples fully resets their charging behavior, which
indicates the underlying global parameter is not intrinsic to the material, but
acquired from its history. Charging behavior is randomly and irreversibly
affected by changes in relative humidity, pointing to a mechanism where
adsorbates, in particular water, are fundamental to the charge-transfer
process.
