The Emiscope 4/1 is an electro-statically focused and deflected tube. EMI developed high definition television in the UK independently of the pioneering work at RCA in the US. Their early picture tubes used the same special base and had magnetic deflection. The 4/1 has a 90 mm diameter screen and was probably used either for a waveform monitor or as an oscilloscope tube.
The special feature of this CRT is the external connections to one set of deflection plates. It is presumed that the Y plates connect this way with the X plates connected via the base. The lower inductance of the side connections would allow for higher frequency waveforms to be displayed, such as the TV video output. A more prosaic explanation could be that the base did not have sufficient pins.
From www.lampes-et-tubes.info comes the operating data. The a(1) voltage would be 160 V, the a2 or final anode voltage would be 800 Volts. The grid voltage swing is 21 Volts. The phosphor is medium persistence green as used on oscilloscope tubes. The X sensitivity with the 800 Volt anode voltage is 0.6 Volts per mm and the Y sensitivity is 0.55 Volts per mm. The similarity of deflection sensitivity also points to the external Y plate connections being to improve HF response.
The screen has a slight dome to it as glass technology of the time would not be strong enough for a flat face to be used without fear of implosion.
The special eight pin base.
The identification label, serial number and at the top the wire connecting the caps to the deflection plates.
The middle of the tube and evacuation pip. The glass is coated with colloidal graphite called Aquadag to act as the final anode.
The base showing the wires to the pinch.
The electron gun at the bottom of the picture and focus anode above. Note the mica rings to maintain alignment within the neck.
A close-up of the electron gun.
The mica spacers and four main support rods.
The end window envelope is 53 mm in diameter, and excluding the special base pins is 400 mm tall.
Reference: www.lampes-et-tubes.info. Type 4/1 was first introduced in 1937. See also1937 adverts.