... touching1
Avoid rubbing which may result in charge separation of opposite sign (depending on the proofball metal and type of insulator: ``triboelectric series'', see exp #6 of Part II).
... fact:2
The actual B at the maximum electron orbit is only $ \sim$ 0.5% less. See Price, ``Electron trajectory in an e/m experiment'', Am. J. Phys. 55, 18, (1987)
... brightness3
CAUTION: a stationary intense spot may damage the screen
... silicon;4
As one might expect since silicon has valence 4, phosphorus valence 5, and aluminum valence 3
... caliper5
In all the following be sure you are not looking through the hole at one end of the jaws. It is advisable to cover the hole with masking tape.
... objective.6
Be sure no light enters the telescope except through the slit jaws. (You may need masking tape on the caliper jaws to prevent this from happening.)