REPEATER DESIGN — NEWFOUNDLAND-NOVA SCOTIA LINKS 263 



that even with stringent precautions mica is not an entirely satisfactory 

 dielectric material. 



Other Comyonents 



Electron tubes form the subject of a separate paper J Of the other 

 miscellaneous components used, one of interest is the short-circuiting 

 fuse across the electron tube heaters. It is constructed like a normal wire- 

 wound resistor on a Sintox tube former, but inside are two cupped copper 

 electrodes filled with a low-melting-point eutectic alloy. If the full line 

 current (316 ma) is passed through the winding, owing to a heater dis- 

 connection, the heat generated is sufficient to melt the alloy, which then 

 fuses the two electrodes together. The winding is thus short-circuited, 

 and a permanent connection is left between the electrodes. 



DESIGN OF HOUSING AND GLAND 



General 



Although the maximum depth of water in which British rigid-type 

 repeaters were laid did not exceed about 250 fathoms, the housings used 

 for these repeaters were generally of a type designed for use at ocean 

 depths, and when connected into the cable they were amply strong 

 enough to transmit stresses up to the breaking point of any of the cables 

 used. 



The part of the housing which is sealed against water pressure consists 

 essentially of a hollow cylinder, machined from hot-drawn steel tube, and 

 closed at both ends by steel bulkheads carrying the cable glands through 

 which the connections are made to the electrical unit (see Fig. 8). The 

 latter is bolted rigidly to the inner face of the A-end bulkhead. The steel 

 blanks used for the main cylinder and the bulkheads are tested with an 

 ultrasonic crack detector, and after machining they are further sub- 

 jected to magnetic crack-detection tests. 



Each gland has a brass cover which completes the coaxial transmission 

 path and contains a weak solid mixture of polythene and polyisobutylene 

 (p.i.b.). Outside the brass cover is a larger chamber closed by a flexible 

 polyvinylchloride (p.v.c.) diaphragm and containing p.i.b. — a viscous 

 liquid — which prevents sea water coming into direct contact with the 

 bulkhead seal and the gland assembly. 



Cylindrical extension pieces, screwed on to the main casing, contain 

 the clamps for attaching the repeater housing to the armour wires of the 

 sea cable, and the housing is completed by dome-shaped end covers. 

 Two external annular ridges near the centre of the housing accommodate 



