# X-ray filament and cathode connection

I was studying how X-ray machines works. I have seen that the filament creates a current that produces electrons. By applying a high voltage between the cathode and anode, the electrons move towards the anode, and x-rays are produced.

I have some issues with the placement of the filament in the tube.

1. Is the filament connected in this manner with the cathode as shown below?
1. I know it isn't possible but if I could somehow connect an smmeter at the battery end as shown below, will I be able to get the current that has passed?

If yes then how? Electrons are produced in the filament and are not flowing from the cathode of the battery to the anode physically. If the answer is no, then how is the current calculated that is passed?

• Why do you say it isn't possible? You can install an ammeter there, it'll just take some care considering the high voltages and low currents involved. Commented Sep 16, 2023 at 21:16
• @Hearth the high voltage is the main issue. And I haven't seen this done somewhere. Commented Sep 17, 2023 at 5:25

1. Is the filament connected in this manner with the cathode as shown below?

Yes

1. Now I know it isn't possible but I somehow connect an Ammeter at the battery end as shown below. Will I be able to get the current that has passed??

Yes. You will be able to measure the current flowing in the loop comprising the two power supplies, the X-ray diode, and the ground connection. It may be safer to install the ammeter in one of the ground connections, then it is at ground potential and so easier to read. It will read the same current as one connected where you show.

If yes then how? electrons are produced in the filament and are not flowing from the cathode of the battery to the anode physically. If the answer is No, then how is the current calculated that is passed??

Electrons are 'boiled off' (a metaphor summarising a lot of physics) the filament due to its high temperature. Electrons ARE accelerated through the electric field towards the anode. X-rays are produced as the electrons hit the anode and decelerate rapidly, releasing most of the kinetic energy they accumulated in their flight to the anode as heat. That's why the anode is shown as a very chunky lump of metal for heatsinking and high thermal capacity, and in some X-ray tubes will actually rotate to spread the heat load along a line on the anode rather than at a point. If you get an X-ray from one of these machines, you will hear a 'wheeee' just before the shot, which is the anode spinning up.

The diode current flows round the entire loop of both power supplies and the X-ray diode.

The cathode shown here is the 'directly heated' type. Most low voltage thermionic tubes use an 'indirect' cathode, where the cathode is electrically insulated from the heater. This means the entire cathode is at the same potential, so the cathode to other electrodes potential difference is consistent. When a cathode is directly heated, there is a potential drop along the filament, and the potential difference to other electrodes changes with position on the filament. With a low voltage tube, that can lead to a large fractional spread of electron energies at the anode. When there's 100 kV across the tube, a few volts difference due to a directly heated cathode is an insignificant fraction.

• So there will be current flowing from the anode to cathode, and that current will be basically due to filament electrons that completes the circuit and makes it a closed one with current in mA flowing from cathode to anode right?? If the filaments one terminal is not connected to the cathode even then we will get a current? Since the electron cloud will still be produced and we can still get the current due to these electrons. Commented Sep 16, 2023 at 19:26
• I usually insist on conventional current, which flows from anode to cathode, but in the case of thermionic valves, the physical passage of electrons from cathode to anode is quite clear and useful. The electrons leave the filament into the electron cloud surrounding it, and are then accelerated towards the anode. Both filament terminals are effectively 'cathode'. There may be a few volts to a few 10s of volts dropped across the filament, nothing compared to the 100s of kV across the diode. If you need a single potential cathode, you use an indirectly heated cathode, insulated from the heater. Commented Sep 16, 2023 at 19:33
• Basically, you have the cathode and anode open circuit but the filament electrons make it a close one. Thats interesting. Sorry for the mistake earlier I meant to say cathode to anode. Commented Sep 16, 2023 at 19:37
• @kam1212: In your drawing, the filament is also the cathode. Commented Sep 16, 2023 at 19:40
• @kam1212: most vaccum tubes that were used in radio and TV receivers and elsewhere had indirectly heated cathodes - the cathode surrounded the filament (usually called a heater in that case) but was electrically insulated from it. I don't know if X-ray tubes ever have indirectly heated cathodes. Commented Sep 16, 2023 at 20:35