In electronics, a vacuum tube, electron tube (in North America), thermionic valve, tube or valve is a device controlling electric current through a vacuum in a sealed container. The container is often thin transparent glass in a roughly cylindrical shape. Within the glass is a vacuum with a cathode and anode at either end. Electrons flow from the cathode to the anode through the vacuum, a demonstration of the Edison effect. The introduction of a grid between the cathode and anode makes it possible to amplify a current. A negative voltage applied to the grid will subdue the current traveling between cathode and anode through the vacuum by a larger value than the current applied to the grid. Hence the grid may be used to modulate the current through the tube.