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shown in the photomicrograph Fig. 15, is about 60% porous. It was pre- 

 pared by pressing iron powder in the Kovar cup and sintering in a hydrogen 

 atmosphere. A special heat treatment to remove oxide made it possible to 

 fill all pores of the sponge with mercury and to supply a mercury film on its 

 surface. Under sparking conditions mercury from this film is evaporated 

 and is condensed on the tube walls, eventually returning to the cathode. 

 Due to capillary action the film is continuously replenished. This film 

 protects the iron sponge from sputtering provided that there is sufficient 



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Fig. 15— Photomicrograph (X15j of a section through a sintered iron sponge showing 

 jjorosity. 



cooling of the cathode to maintain the mercury film at a temperature below 

 its boiling point. 



These gaps are not temperature sensitive as are most electronic devices 

 contaming mercury. This is because the mercury vapor plays no essential 

 role in the spark discharge, as indicated by the fact that dissipation measure- 

 ments—discussed in II- (e)— show its dependence on the hydrogen-argon 

 rather than on the nature of the cathode material. With adequate cathode 

 cooling gaps of this type operate satisfactorily over a range of ambient tem- 

 perature at least from -50°C to over 100°C. Practical gaps constructed 



