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452 RICHARDSON'S BLUEBOOK OF PROJECTION
its initial velocity. The filament has become positive to the extent of the loss of one negatron. This positive charge of the filament exercises an attractive force upon the negative electron, slowing down its motion until it is brought to a full stop. The same attraction then draws the negatron back' into the filament from which it had escaped.
(4) Now, if we consider the multitudes of negatrons emitted from the filament, it is clear that the filament will always, while it remains incandescent, be surrounded by a cloud of negatrons. Some will be just coming out of a filament. Some will have moved away from the filament to the maximum distance they can attain. Some will be moving back towards the filament and re-entering it. This cloud of negatrons surrounding an emitting surface constitutes a negative charge in the area of space or vacuum which it occupies. When it occurs within a vacuum tube, this charge is called a "space charge/'
The practical uses of emitting filaments and of the space charge in vacuum tubes will be discussed on a later page. Here we are concerned only with incandescent metals as sources of negatrons.
(5) But it is very necessary to note that it is the metal which is the source of the negatrons, and not the current that is passed through it to heat it. In order that there may remain no doubt upon this point it may be desirable to pause now to describe the "heater" type of filament used in a number of vacuum tubes. The construction of this filament clearly shows that the incandescent metal and not the heating current is the source of emitted negatrons.
Heater Filaments
(6) The heater filament is found, for example, in the "27" type amplifying tube which is used in many sound systems and most radio receivers. The filament consists of a very thin, hair-pin shaped wire. This wire passes through, and is surrounded by, a substance which is an insulator of electricity but a moderately good con