64 
The Queensland Naturalist 
April, 1930 
up for use of these tubes. This tube has a very high 
vacuum, and no free moving electrons exist as there are 
no atoms present. No ionisation in an electron tube can 
take place, hence also it is not possible to pass an electric 
current through a complete vacuum. However, it is pos- 
sible to force electrons across space by heating a so-called 
filament. 
A few short remarks in regard to the modern concep- 
tion of the electron theory may not be amiss. The atom 
is to be compared with a solar system, having a main 
stationary centre, which is the positive portion of the 
atom. The remainder of the atom, equal in electrical 
value, is made up of small electric charges named elec- 
trons; they are negatively charged, and are able to move 
freely, and are in no # way dependent on the atom from 
which they come. 
It is the electron, the negatively charged particles, 
that respond to electric forces; wherever an electric 
current is flowing we have wandering electrons. When a 
body has a negative charge, then such a body has a surplus 
of electrons, and vice-versa, a positive charge has a defi- 
ciency of electrons. A good conductor of electricity is 
such where the electrons can move about with great ease, 
whereas in a bad conductor the movement of electrons 
is restricted. Heat assists the free movement, as we have 
noted that when the air becomes incandescent it makes a 
good conductor. 
The Electronic X-Ray tube contains a filament 
so arranged that it can be heated up by a low voltage 
electric current. A variation in the strength of this 
current will also vary the heating effect of the filament 
wire. In turn more or 'less electrons are liberated to be 
driven across to the target when the high voltage current 
is applied. 
It is the electron supply that governs the high-tension 
current and is practically independent to voltage. Low 
voltage produces a soft ray, or in other words, a ray that 
is readily absorbed, and its penetration properties through 
substance is small. High voltage, however, produces a 
more penetrating ray. The course of variation in pene- 
tration or quality of the ray is the speed at which the 
electrons are driven from the filament to the anode. 
The quality of the ray is of great importance, and is 
independent to the quantity of current, which is, as before 
said, dependent on the electrons available at the cathode. 
The voltage required to produce X-Rays of suitable 
quality for diagnostic purposes ranges from 40,000 to 
