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) between cathode and anode, 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 developed between 1907 and 1914 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.