Diode

One of the simplest devices made By combining some elements placed inside a vacuum tune or evacuated tube is the diode. It contains two metal electrodes placed directly opposite each other inside a vacuum. One of these metal electrodes is taken as emitter of elections, I.e. cathode, which will emit or radiate electrons into the vacuum, while the other plate placed at the opposite end of the vacuum tube, called 'anode', is to collect all those electrons that are emitted by the emitter. The vacuum tune in which these cathode and anode are placed is made of a highly evacuated envelope of either glass or metal. We recall, from our previous studies, that two bodies having the same charge will repel or move away from each other, while two bodies which have different or unlike charges will attract of move closer to each other. This is exactly what happens in a diode. Since the two electrodes in the diode are placed in a perfect vacuum, with one of these electrodes streaming out electrons while the other is catching them, it means that the vacuum will be filled with some quantity of electrons which when they become very many, will start to form a cloud or clusters. These electrons will have now where to go but stay within the vacuum, which is being heated, and since they are of the same negative charge, they will start to repel, I.e. move away from each other until they occupy the whole space of the vacuum tube.

Also , we have learnt that these electrons ate the current carriers, provided they are made to move by a force known ad voltage from a potential difference. Their motion helps current to be carried from one part to other. For electrons packed in to the vacuum to move at all, they need some quantity of force to either push or pull them. This is made possible by the simple method of letting one electrode intended as the catcher of collector to attain charges that are opposite to those of the electrons to be attracted; that is to say that the collector electrode will have to be of positive charge. When this is the case, the electrons will quickly move from the vacuum to the positively charged electrode (the collector). This movement of electrons will continue from the cathode (electron producer or the emitter) to the positively charged electrode or anode (the electron collector).

    By letting this movement of electrons (migration ) continue , there will come a time when all the electrons will move out of the cathode, and a the positive charge on the anode will diminish or finish, then there will be no means of electron movement. Thus, there should be a provision of means by which the movement of these electrons will continue indefinitely so that if we should connect an ammeter between these two electrodes, one will notice the presence of electric current.

    For the process described  above to continue, we need to let the cathode be supplied with electrons while the anode is given the positive charge continuously. This is usually done by making the electrons which are   moving out of the vacuum tube through the anode to come back to the cathode and continue the motion (cycle), while the positive charge on the anode will still  remain In their position. The simplest method of doing this is by providing a potential different source from a battery. This is done by connecting the battery terminals to the cathode and anode. At this point, the positive terminal of the battery is connected to the anode while the negative terminals is connected to the cathode. If an ammeter is connected between these electrodes. Then there will be a deflection indicating the quantity of current passing through the space between the two electrodes from cathode to anode. If the battery terminals are connected in opposite ways, i.e. the negative terminal to the anode and the positive terminal to the cathode. There would be no movement of the electrons at all And the ammeter will not indicate any passage of current. Notice that the electrons, while the circuit is on, are moving from the cathode to the anode in the vacuum tube, without any disturbance or obstacle. this continues through the ammeter, battery and back to cathode again to repeat their (electrons) journey. The current moving moving out of the anode of the diode is called anode current. The electrons can only  move from the cathode to the anode and not from the anode to the cathode. This means that the electrons can only move in one direction; so also is the anode current.

   Due to these peculiarities associated with the diode, it serves ad s rectifier which  will allow current to move towards one direction only. thus, if alternating current (AC) is applied to the diode, the output will be director current (DC).