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518 T. M. C. LANCE November
of the projection tubes under the arduous conditions called for by the specification. The tube produces 900 1m of luminous flux at 50 kv and a beam current of 5 ma. On a television picture the average current is generally about 1-1.5 ma. with peaks between 10-15 ma.
Figure 4 shows the external appearance of the projection tube alongside the focus and deflection unit previously described. The optically polished and curved face, the double getters and the electrode system can be easily recognized.
The main body of the tube is a mould-blown glass bulb, and all the parts are constructed of the same boro silicate glass manufactured by Chance Bros, and known as "Hysil." The face is sealed to the bulb in a special jig, gas fires being used in the normal manner. All the glass is given a preliminary E.H.T. test to double the working voltage,
Fig. 4. Projection tube and coils.
as many failures have been traced to minute bubbles in the neck ionizing under working conditions and bursting.
Electrode System
The tube is a simple triode with the anode and modulator carefully cleaned and polished to reduce point discharges and cold emission at 50 kv.
The main problem was that of obtaining sufficient emission to give a focused beam current of 15 ma, and of the many cathode structures tested the most satisfactory was the flat top filament tape of pure tungsten. This was superior in that the unfocused beam was substantially circular, and hence made better use of the available focusing and deflection aberration-free areas, and also the spot passed through focus in a more symmetrical manner.