  The existence of volcanic lightning and alteration of the atmospheric
potential gradient in the vicinity of near-vent volcanic plumes provides strong
evidence for the charging of volcanic ash. More subtle electrical effects are
also visible in balloon soundings of distal volcanic plumes. Near the vent,
some proposed charging mechanisms are fractoemission, triboelectrification, and
the so-called "dirty thunderstorm" mechanism, which is where ash and convective
clouds interact electrically to enhance charging. Distant from the vent, a
self-charging mechanism, probably triboelectrification, has been suggested to
explain the sustained low levels of charge observed on a distal plume. Recent
research by Houghton et al. (2013) linked the self-charging of volcanic ash to
the properties of the particle size distribution, observing that a highly
polydisperse ash distribution would charge more effectively than a monodisperse
one. Natural radioactivity in some volcanic ash could also contribute to
self-charging of volcanic plumes. Here we present laboratory measurements of
particle size distributions, triboelectrification and radioactivity in ash
samples from the Gr\'{i}msv\"{o}tn and Eyjafjallaj\"{o}kull volcanic eruptions
in 2011 and 2010 respectively, and discuss the implications of our findings.
