An indicator tube is an electron ray device, with a
fluorescent, transparent screen instead of the normal anode plate. In addition
to the usual grid and cathode electrodes, it includes a small electrode, called
a target and usually made of a thin wire, whose potential affects the deflection
of the electron beam and thus determines the width of a lighted area on the
screen. The grid is usually internally connected to the cathode to safely limit
the screen current.