ather their individual atoms are arranged. Sometime you may wish to
read how this was found out by the use of X-rays.[6] Take the crystal of
common salt for example. That is well known. Each molecule of salt is
formed by an atom of sodium and one of chlorine. In a crystal of salt
the molecules are grouped together so that a sodium atom always has
chlorine atoms on every side of it, and the other way around, of course.

Suppose you took a lot of wood dumb-bells and painted one of the balls
of each dumb-bell black to stand for a sodium atom, leaving the other
unpainted to stand for a chlorine atom. Now try to pile them up so that
above and below each black ball, to the right and left of it, and also
in front and behind it, there shall be a white ball. The pile which you
would probably get would look like that of Fig. 69. I have omitted the
gripping part of each dumbell because I don't believe it is there. In my
picture each circle represents the nucleus of an atom. I haven't
attempted to show the planetary electrons. Other crystals have more
complex arrangements for piling up their molecules.

Now suppose we put two different kinds of substances close together,
that is, make contact between them. How their electrons will behave will
depend entirely upon what the atoms are and how they are piled up. Some
very curious effects can be obtained.

[Illustration: Fig 70]

The one which interests us at present is that across the contact points
of some combinations of substances it is easier to get a stream of
electrons to flow one way than the other. The contact doesn't have the
same resistance in the two directions. Usually also the resistance
depends upon what voltage we are applying to force the electron stream
across the point of contact.

The one way to find out is to take the voltage-current characteristic of
the combination. To do so we use the same general method as we did for
the audion. And when we get through we plot another curve and call it,
for example, a "platinum-galena characteristic." Fig. 70 shows the
set-up for making the measurements. There is a group of batteries
arranged so that we can vary the e. m. f. applied across the contact
point of the crystal and platinum. A voltmeter shows the value of this
e. m. f. and an ammeter tells the strength of the electron stream. Each
time we move the slider we get a new pair of values for volts and
amperes. As a matter of fact we don't get amperes or even mil-amperes