<P> Since the electrons have a negative charge, they are repelled by the negative cathode and attracted to the positive anode . They travel in straight lines through the empty tube . The voltage applied between the electrodes accelerates these low mass particles to high velocities . Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called fluorescence . Researchers noticed that objects placed in the tube in front of the cathode could cast a shadow on the glowing wall, and realized that something must be travelling in straight lines from the cathode . After the electrons reach the anode, they travel through the anode wire to the power supply and back to the cathode, so cathode rays carry electric current through the tube . </P> <P> The current in a beam of cathode rays through a vacuum tube can be controlled by passing it through a metal screen of wires (a grid) to which a small negative voltage is applied . The electric field of the wires deflects some of the electrons, preventing them from reaching the anode . The amount of current that gets through to the anode depends on the voltage on the grid . Thus, a small voltage on the grid can be made to control a much larger voltage on the anode . This is the principle used in vacuum tubes to amplify electrical signals . The triode vacuum tube was the first electronic device that could amplify, and is still used in some applications such as radio transmitters . High speed beams of cathode rays can also be steered and manipulated by electric fields created by additional metal plates in the tube to which voltage is applied, or magnetic fields created by coils of wire (electromagnets). These are used in cathode ray tubes, found in televisions and computer monitors, and in electron microscopes . </P> <Ul> <Li> <P> Crookes tube </P> </Li> <Li> <P> Cathode rays travel from the cathode at the rear of the tube, striking the glass front, making it glow green by fluorescence . A metal cross in the tube casts a shadow, demonstrating that the rays travel in straight lines . </P> </Li> <Li> <P> A magnet creates a horizontal magnetic field through the neck of the tube, bending the rays up, so the shadow of the cross is higher . </P> </Li> <Li> <P> When the magnet is reversed, it bends the rays down, so the shadow is lower . The pink glow is caused by cathode rays striking residual gas atoms in the tube . </P> </Li> </Ul> <Li> <P> Crookes tube </P> </Li>

Cathode ray when obstructed by metal cause emission of