The third form is that the cathode has a heater so that the cathode temperature is controlled in a range of {TAIL ~ the room temperature} to about 100 degrees Celsius to regulate an amount of electron emission from {HEAD ~ the cathode} 
The third form is that the cathode has a heater so that the cathode temperature is controlled in a range of { TAIL ~ the room temperature } to about 100 degrees Celsius to regulate an amount of electron emission from { HEAD ~ the cathode }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD