With the help of thermionic emission, electrons are emitted by a heated cathode, and then accelerated toward a grid which is at a positive potential, relative to the cathode. The collecting plate is at a lower potential and is negative with respect to mesh grid. If electrons have sufficient energy on reaching the grid, some will pass through the grid, and reach collecting plate, and it will be measured as current by the ammeter. Electrons which do not have sufficient energy on reaching the grid will be slowed down, and will fall back to the grid.The experimental results confirm the existence of discrete energy levels.