The photomultiplier has a light sensitive electrode called the
photocathode
which emits electrons when photons strike it. These
electrons are accelerated
by a series of electrodes , called dynodes
,
towards a collector (or anode ). The
electrons produce several secondary electrons each
time they strike a dynode
resulting in a multiplication of their number as they approach the anode.
Fig.
shows a photomultiplier tube. An external resistor chain
connected to a stable power supply is used to produce the voltages which are
applied to the dynodes.
Typically, each electron which strikes a dynode
will produce about four secondary electrons . This
means that if one electron is released
from the photocathode , a phototube with 10 dynodes will
deliver 
electrons to the collector. This gain, of about one
million, is critically dependent upon the dynode voltages which necessitates
a very stable power supply.
