We demonstrate that the walls of glass cells become electrically conductive
when exposed to a dry cesium vapor, glasses of different compositions lead
ing to similar effects. We find dramatically different results for monocrys
talline sapphire cells, which exhibit typical resistances of a few G Omega,
nearly five orders of magnitude larger than glass cells in similar conditi
ons. In spectroscopy experiments requiring the application of an electric f
ield, low cell resistances imply leakage currents generating stray magnetic
fields. The latter, at the origin of harmful effects in precise tests of t
he fundamental laws of physics performed in cesium cells, will thus be supp
ressed in sapphire cells. Moreover, with such cells it becomes possible to
place the electrodes outside.
A tentative interpretation of the surface electrical conductivity of glass
cells is propounded by establishing a connection with the surface coverage
by cesium atoms physically adsorbed on the glass surface. This results from
the observed dependences of the cell conductance versus Ca vapor density i
n the mtorr range and versus the wall temperature up to 200 degrees C, whic
h indicates an activation energy of 0.66 +/- 0.05 eV. In the sapphire cell,
where there is no hint of surface effects, the conductivity looks instead
attributable to collisional processes occuring inside the vapor for Ca numb
er densities greater than or similar to 10(14) at/ cm(3).