In experiments EF-1 and EF-2 you will study the motion of charged particles (electrons) in electric and magnetic fields. The electrons behave as classical particles whose behavior is governed by Newton's laws of motion. That is, the speeds are always small compared to the speed of light so that no relativistic correctons are needed, and the dimensions of the experiment are large compared with atomic dimensions, so quantum effects need not be considered.
The central instrument in these experiments is an electron beam tube whose operation is quite similar in principle to that of a television picture tube. The tube is commonly called a cathode-ray tube (CRT). This name originated in the mid-nineteenth century in the course of investigations of the conduction of electricity through gases at low pressures. In such experiments, bluish rays were emitted from the region near the cathode and were called "cathode rays". In addition to providing a convenient experimental arrangement for the study of electron motion, the cathode-ray tube is also an important component of the cathode-ray oscilloscope which will be used in many later experiments in this and in other courses.
A cathode-ray tube contains (1) an electron gun, which emits
electrons, accelerates them to a definite speed, and focuses them
into a beam; (2) a deflection system consisting of two pairs of
plates; and (3) a fluorescent screen to indicate the point of impact
of the elecron beam on the end of the tube. All these are enclosed
in a glass envelope which is evacuated so that the electron beam
will not be scattered by air molecules. In the tube the air pressure
is reduced to about 
atm. The arrangement of components is
shown in the figure below.
=-3in =-13 The electron gun is shown in more detail in the figure at right. The source of electrons is the cathode, labeled K in the figure. A thin cylinder is heated to about 1200 K by passing a current through a twisted heater wire inside the cylinder. The wire is insulated from the cylinder by a ceramic sleeve. The end of the cylinder forming the cathode is coated with barium and strontium oxides; when these materials are heated, some of the electrons in them acquire enough energy to break away and thus become free to move in the vacuum surrounding the cathode. This process is called thermionic emission.
Coaxial with the cathode are four cylinderical electrodes
containing baffles with
circular apertures, as shown. Electrode 
,
called the control grid, is operated at a potential of 1 to 5
volts negative with respect to the cathode; the resultng electric
field tends to push electrons back toward the cathode. Hence by
varying this potential it is possible to controll the number of
electrons emerging from
the aperture of 
, thus controlling the intensity of the beam.
Electrode 
is connected internally to 
, and
both are operated at a potential 
of several hundred volts
positive with
respect to K. The resulting field accelerates electrons
along the axis of the gun. Electrode 
is held at a
potential 
(with respect to K)
intermediate between those of K and 
. The resulting fields
between 
and 
and between 
and 
serve
to focus the beam, so that electrons emerging from 
in various
directions are brought together in a small beam whose diameter is
determined mainly by the size of the aperture in
. Proper focusing
depends chiefly on the ratio 
.
The electron beam passes through two pairs of deflection plates shown in the first figure. A potential difference applied between the two plates of either pair produces a transverse electric field which deflects the beam sideways; this deflection is studied in detail in the first experiment. Finally, the beam strikes the phosphor-coated end causing a visible spot on the screen. This results from collisions of the electrons with atoms of the phosphor, raising them to energy levels above the gound state. When the atoms return to their ground states, they release energy in the form of visible light.
The inner surface on the sides near the front of the tube is
coated with
a graphite conducting material (Aquadag). It is electrically
connected to the second anode 
, and thus serves as an extension
of it; it also helps to shield the beam from external electric
fields. It collects the secondary electrons emitted from
the phosphor by electron
bombardment, thus completing the electrical circuit
for the beam.
CAUTION
Because of the high vacuum and the large flat surface
area of the screen face, the tube is dangerous to handle.
Any weakening of the glass envelope which might
result from mechanical shock or a scratch may cause a
severe explosion, resulting in a lot of flying glass.
The tube should be kept in its plastic cover and not
handled while wearing a diamond ring which could
scratch the glass.