As a result, electrons emitted from the cathode are repelled away from the grid, back towards the cathode, and so are prevented from reaching the anode. With a small positive bias applied to the grid, electrons pass through the grid, towards the anode, and the process of establishing an arc discharge can commence. However, once the arc has been established, it cannot be stopped by grid action, because the positive mercury ions produced by ionisation are attracted to the negatively charged grid and effectively neutralise it.