194. ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
balance of the atom while the nucleus exerts the “positive” force which 
holds it together; and we also know that when this balance is upset, 
due to gain or loss of electrons, we think of the atom as “charged,” 
since it is this circumstance which causes the tiny particle to attract or 
repel other electrons according to the state of its unbalance. And 
science has succeeded in unbalancing the atoms to such an appreciable 
extent that the negative electricity may be withdrawn and harnessed 
for use in such instruments as the electron microscopes. 
The fact has long been established that atoms are in a constant 
state of vibration in a heated body and that the greater the heat of the 
body, the greater the agitation of the atoms. According to the electron 
theory of metals, electrons circulate about a three-dimensional network, 
or lattice, of positive ions, some of the electrons being comparatively 
free, that is to say, the attractions of the ions are practically canceled 
by the repulsions of the other electrons. It does not necessarily follow, 
however, that the same electrons consistently remain free. They may 
be controlled by the ions eventually, but regardless of this, there is 
always a fixed number of them that are free. Moreover, there is a 
critical value of speed above which the electrons are able to rise in 
metals and thus escape from their restraining positive charges, though 
at ordinary temperatures the proportion of them moving rapidly 
enough to do this is relatively small. However, as the heat applied 
to the metal is increased, not only is the thermal agitation of the 
electrons increased also, but the proportion among them possessing 
sufficiently high speeds to enable them to leave the metal. 
Thus is heat applied to the electron source of the electron micro- 
scope which, in the case of most instruments of this kind, is a tungsten 
filament surrounded by a guard cylinder. After leaving the filament, 
or cathode, the electrons enter an electric field wherein are large 
accumulations of charge which serve to speed up steadily the motion 
of these freely moving particles. Since the electrons travel in vacua, 
none of the kinetic energy gained in crossing the field is lost, the total 
kinetic energy, or energy of motion, gained in passing through this 
region being proportional to the voltage applied. We may deduce, 
therefore, that since increase of charge in an electric field means a 
proportional increase of kinetic energy of these electrons, the higher 
the voltage applied, the greater the speed of the electrons—all of 
which has been calculated mathematically and confirmed experi- 
mentally. 
After traversing the electric field and passing through the anode, 
the electrons are concentrated on the specimen under examination by 
the first of three magnetic fields which are created by currents flowing 
through coils enclosed in soft iron shields, molded so as to concentrate 
