Secondary emission
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Secondary emission is a phenomenon that occurs in electron tubes where electrons impact an electrode with sufficient energy to 'knock' additional electrons from the surface of that electrode. Generally, one electron gives rise to several secondary electrons.
This effect can be undesireable such as in the tetrode thermionic valve (tube). In this instance the positively charged screen grid can accelerate the electron stream sufficiently to cause secondary emission at the anode (plate). This can give rise to excessive screen grid current. It is also partly responsible for this type of valve (tube) exhibiting a 'negative resistance' characteristic.
The effect can also be exploited to advantage such as in the photomultiplier tube. In this instance the electrons (or indeed electron) emitted from a photocathode are accelerated towards a polished metal electrode (called a dynode). This electron or electrons strike with sufficient energy to 'knock' many more electrons from its surface. These new electrons are then accelerated towards another dynode where even more electrons are emitted. This process occurs (typically) 10 or so times. The result is that the tiny and normally undetectable current from the photocathode becomes a much larger and easily measurable current flowing in the final anode circuit. The current gain is typically many hundreds of millions.
In the 1930s special amplifying tubes were developed which deliberately "folded" the electron beam, by having it strike a dynode to be reflected into the anode. This had the effect of increasing the plate-grid distance for a given tube size, increasing the transconductance of the tube and reducing its noise figure. A typical such "orbital beam hexode" was the RCA 1630, introduced in 1939. Because the heavy electron current in such tubes damaged the dynode surface rapidly, their lifetime tended to be very short compared to conventional tubes.