Why evacuate the glass tube in Thomson's e/m ratio experiment

[M. next]

This experiment aims towards finding e/m ratio and it is a very well known experiment that's why I will directly post my question. Why do we evacuate the glass tube?

 

[technician]

electrons would be easily stopped in a tube containing gas.

 

[Doc Al]

Why do we evacuate the glass tube?

The better the vacuum, the better the electrons can travel unimpeded by collisions with gas molecules. A beam of electrons won't get far in air before being scattered.

 

[M. next]

Uh, I see.. Great thanks.