You have a horizontal cathode ray tube (CRT) monitor for which the controls have

Question

You have a horizontal cathode ray tube (CRT) monitor for which the controls have been adjusted such that the electron beam should make a single spot of light at the exact center of the screen. You observe, however, that the spot is deflected to the right. It is possible that the CRT is broken. But as a clever scientist, you realize that your room might be in either an electric field or a magnetic field. Assuming you have no tools except the CRT monitor, how can you determine whether the CRT is broken, in an electric field, or in a magnetic field?

Explanation / Answer

You can rotate the CRT half a turn (so it faces the other way). If there is an external magnetic field, then the spot should now be deflected the other direction (i.e. a little bit to the left instead of to the right). If the deflection changes direction when the CRT faces the other way, then its being caused by something in the room and the CRT is not broken. If the spot remains deflected in the same direction (i.e. it stays to the right) then the CRT is broken.

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