why it was necessary to take low pressure inside the cathode ray tube when the exoeriment was going on?

@ gaurav 
this is beacuse most of the gases donot conduct electricity at normal pressure , , so, at decreased pressure they will be able to conduct electricity , moreover 
it is also reduced to prevent the loss of energy at cathode ray when they travel in a discharge tube . 
the gas can conduct electricity at low pressure only which is between 0.1 – 0.001 mm / hg 
with very high potential diffrence which is about 10,000 V 
hope it clears your doubt 

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