1312 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



other hand, if there is appreciable potential difference between helix 

 and collector, we must insure that few secondary or reflected electrons 

 return from the collector to bombard the helix and accelerator, or else 

 we may overheat these electrodes. Fig. 21 shows a drawing of the col-- 

 lector used in the M1789. It takes the form of a long hollow cylinder 

 shielded from the magnetic field. Inside of the collector the beam is 

 allowed to gradually diverge and the electrons strike the walls at a graz- 

 ing angle. This design reduces secondary electrons returned from the 

 collector to almost negligible proportions. 



^CYLINDRICAL 

 ' GLASS SLIDE 



PHOTOCELL 



Fig. 20 — Schematic diagram of the machine used for .spra.ying aquadag attenu- 

 ation on the helix. In this machine the helix is rotated rapidly to insure uniform 

 exposure to the spray. At the same time the masking drum rotates at a slower 

 speed and the spra}- gun traverses back and forth along the masking drum. The 

 drum therefore acts as a revolving shutter between the helix and the spray gun 

 and its degree of opening serves to control the amount of aquadag reaching the 

 helix. From a knowledge of the rate of attenuation increase as a function of the 

 amount of carbon deposited (empirically determined) the shape of the drum open- 

 ing can he calculated so as to give any desired attenuation pattern. 



The spray gun also passes over a glass cylinder at one end of the masking drum 

 so that it receives a sample of the aquadag spray. A photocell is used to monitor 

 light transmitted through the cylinder. Before starting to spray, the glass is 

 cleaned and the photocell reading is taken as 100 j)er cent light transmission. 

 The helix is then sprayed until the light transmission has decreased to the proper 

 value. The photoelectric monitoring techniciue makes the attenuation added in- 

 sensitive to the aquadag composition and to the spray gun nozzle opening. 



