Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube. To release electrons into the tube, they first must be detached from the atoms of the cathode. In the early experimental cold cathode vacuum tubes in which cathode rays were discovered, called Crookes tubes, this was done by using a high electrical potential of thousands of volts between the anode and the cathode to ionize the residual gas atoms in the tube. The positive ions were accelerated by the electric field toward the cathode, and when they collided with it they knocked electrons out of its surface; these were the cathode rays. Modern vacuum tubes use thermionic emission, in which the cathode is made of a thin wire filament which is heated by a separate electric current passing through it. The increased random heat motion of the filament knocks electrons out of the surface of the filament, into the evacuated space of the tube.