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The construction of a seal using horn silver is relatively simple. The diagram shows a lead through for an electrical connection
into a vacuum system. An electrode C is brazed to a wire D. D must be a silver or platinum wire for silver chloride reacts with stainless steel,
iron and other metals. A short section of borosilicate capillary B is sealed to the tube A. The region of the joint is packed with powdered
Silver Chloride or Silver Chloride/Lead Chloride Eutectic mixture. the lower end of capillary B can be sealed with a little ceramic cement to
prevent the horn silver escaping when melted.
The whole assembly is placed in an oven and heated to 500 deg C in the
case of pure Silver Chloride, or 350 deg C in the case of the eutectic mixture. This temperature is maintained for several hours and then the
oven slowly cooled over a 10 hour period. Rapid cooling can result in slight leaks in the horn silver joint.
The joints were tested by constructing a small discharge tube using them. The diagram shows the tube and the discharge obtained with a small
induction coil connected across it with air at 1mm Hg internal pressure.
The main features of the discharge are: R the cathode glow, S, the Crookes Dark Space, T, the Negative Glow, U the Faraday Dark Space, and V the Positive
Column. Of these, the Crookes Dark Space is most difficult to observe but can be just discerned in the photograph below.
