INVESTIGATING NEW RAYS
- Dalton, in 1808 proposed that matter is made of
atoms. All substances were either made of single atoms or
combinations of atoms (molecules).
- He thought that atoms were indivisible.
- In the 19th century, experiments showed that atoms were
divisible. As a result, new particles and forces were found.
- Geissler in the 19th century, Invented a new
- He produced discharges of electricity in evacuated tubes
of varying shape. He also produced difference colors of discharge by
placing different gases in the tubes.
GAS DISCHARGE TUBE - A tube that allows an electric current to
pass through a gas at low pressure.
ELECTRODES - Metal plates sealed in the ends of a gas
discharge tube. (+ is the anode and - is the
- When air is pumped out of the tube, the discharge across
an induction coil stops, and the electrodes in the tube are connected by
one or more violet streamers.
- At low enough pressure, a pink glow fills the entire
tube. Continued decreases in pressure cause the pink glow to
concentrate around the anode and a blue glow around the cathode. The space
between the glows is dark (called Faraday's dark space).
- Continued reduction in pressure causes the dark space to
expand, and the color at the electrodes to fade until the tube is dark,
except for a faint green or violet glow around the anode. The sides of the
tube fluoresce (usually green). The dark region is now called Crookes'
- Investigations centered on what was happening in the dark
space. Early researchers decided that the glow in the gas originated at
the cathode. For this reason, the discharge was called CATHODE RAYS, and
the tube a CATHODE RAY TUBE (CRT).
MALTESE CROSS TUBE
- Plucker made an anode into a Maltese cross, and this
produced a shadow in the glow at the end of the tube. This showed that the
cathode rays traveled in straight lines.
PADDLE-WHEEL DISCHARGE TUBE
- Crookes reported that a paddle wheel placed in the path
of the cathode rays turned. This proved that the cathode rays carried
energy, and that they might be made of particles.
- This also indicates that the rays (particles) moved from
the cathode to the anode.
CATHODE RAYS IN A MAGNETIC FIELD
- Crookes showed that the rays were deflected by a magnetic
- Crookes noted that charged particles in a magnetic field
experience a force. Cathode rays behaved as if they were
negatively charged particles.
CATHODE RAYS IN ELECTRIC FIELDS
- Arthur Schuster noticed that the particles were repelled
from a negative plate and attracted to a positive plate.
- This is further proof that cathode rays are negatively
CATHODE RAYS CARRY A NEGATIVE CHARGE
- Jean Perrin constructed an apparatus that had an anode
made of a hollow aluminum cylinder that was open at both ends. At the end
opposite to the cathode, was a cylinder that was closed at one end.
- The closed cylinder collected the cathode rays and was
connected to an electroscope, which was used to determine the charge on
the cathode rays.
- The electroscope showed that the cathode rays were negatively
- The deflecting plates deflected the particles in one
- Magnetic coils deflected the particles in the other
- BY adjusting the relative strength of the electric and
magnetic fields, the particles went straight.
- From measurements and equations for deflecting particles
by magnetic and electric fields, the charge to mass ratio was determined
(1.76 X 1011 C/kg).
- The charge to mass ratio (Q/M) was the same regardless of
the potential difference used to accelerate particles.
- Q/M was the same for different cathode materials.
This indicates that there must be a similarity between particles making up
different cathode materials.
- Similar experiments with hydrogen ions, showed that the
hydrogen lon's charge to mass ratio was 1836 times
smaller than for cathode rays.
- If we assumed that equal charges were present on the
hydrogen ions and cathode ray particles, then the mass of the cathode ray
particles was 1/1836 of the mass for the hydrogen ion.
- The hydrogen atom is the smallest atom, yet these
particles were smaller. This meant that the atom was not the smallest
- Thomson concluded that cathode rays were light, fast
moving, negative particles, that were a part of an atom.
- Thomson called the cathode ray particle, the ELECTRON.
- They are produced by the negative electrode, or
cathode, in an evacuated tube, and travel towards the anode.
- They travel in straight lines and cast sharp
- They have energy and can do work.
- They are deflected by electric and magnetic fields and
have a negative charge.
- They are beams of tiny, negatively charged particles