Publisher Summary This chapter focuses on measurement of the probability distribution P(n) for the number n of secondary electrons emitted when a primary electron strikes a transmission-type electron-multiplying dynode. In the method applied, each group of secondary electrons arising from a single primary is accelerated and focused on to a semiconductor barrier-layer detector, where they are absorbed within the sensitive layer effectively simultaneously. The resulting pulse of charge at the detector output, which is proportional to the total energy absorbed, is therefore proportional to the number n of secondaries. The results show that the most likely outcome of a primary encounter, if there is any emission at all, is always the emission of a single secondary; and the chances of the emission of larger numbers of secondaries then follow in descending, and approximately geometrical, progression. This result does not fit easily into the framework of the customary model of the secondary emission process. This model, which can be made to explain well enough the observed dependence of yield on primary energy, assumes that the dissipation of this energy leads to the production of secondary electrons in the interior of the dynode, each of which diffuses and may escape from the surface.