174 



BELL SYSTEM TECHNICAL JOURNAL 



angles to a magnetic field. The tape, 6 mils wide and 0.3 mil thick, 

 is secured to windlasses A and A' and stretched tight by the spring 

 held pulley B. At points C and C insulated pincers confine the 

 central portions of the tape between windlasses and pulley to form a 

 slit 2 mils wide. Supporting this loop and adjusting devices is a 

 slab of metal with central elevation D, which constitutes the armature 

 of an electromagnet. The central portions of the loop are supported 

 on insulating bridges to lie 3 mils above the face of D; here the sides 

 of the loop are centered over a tapered slot, 8 mils wide by 256 mils 

 long in this plane, opening to 204 mils by 256 mils at the outside face 

 of the armature. Viewed against the light, the valve appears as a 

 slit 2 mils by 256 mils. 



Fig. 1 — The light valve. 



The electromagnet core has a similar elevation opposing D across 

 an air gap of 8 mils which closes to 7 mils when the magnet is energized 

 from a 12 volt battery. A tapered slot in the magnet core begins 

 8 mils wide by 256 mils long and opens with the same taper as the 

 slot in the armature. When the assembly of magnet and armature is 

 complete, the valve constitutes a slit 2 mils by 256 mils, its sides 

 lying in a plane at right angles to the lines of force and approximately 

 centered in the air gap. The windlasses A and A', one of which is 

 grounded, are connected to the output terminals of the recording 

 amplifier. If the magnet is energized and the amplifier supplies a 

 sine wave current from an oscillator, the duralumin loop opens and 

 closes in accordance with the current alternations. 



When one side of the wave opens the valve to 4 mils and the other 

 side closes it completely, full modulation of the aperture is accom- 



