BALANCED POLAR MERCURY CONTACT RELAY 1395 



the glass at the upper end. The fixed contacts are made from small 

 platinum alloy balls which are welded to the pole-pieces. The armature 

 strikes the fixed contacts at a point which is close to a node for one of 

 its principal modes of vibration. This limits the bounce on impact to an 

 amphtude which does not open the mercury bridge which is formed. 



Mercury in a pool at the bottom of the switch is fed to the contact area 

 through grooves rolled in the armature surface. Except for the platinum 

 alloy contacts, the surfaces of the pole-pieces inside the switch have an 

 oxide coating which prevents them from being wet by the mercury. This 

 limits the mercury bridge which is formed between the armature and 

 pole-piece to the area of the platinum alloy contact. A ceramic detail is 

 inserted between the pole-pieces at the top of the switch. The ceramic is 

 specially resistant to wetting by the mercury and thus prevents mercury 

 from collecting between the pole-pieces. 



The polarizing magnets are soldered to the pole-piece terminals out- 

 side the switch. Permalloy plates are soldered to the outer poles of the 

 magnets and extend down on the outside of the coil, forming a return 

 path to the lower end of the armature. The coupling at the lower end is 

 made relatively loose in order to limit magnetic soak effects. The steel 

 can provides a magnetic shield. 



STATIC MERCURY CONFIGURATION 



The static configuration of the mercury surfaces in the switch depends 

 upon the shape and contact angle to mercury of the solid surfaces with 

 which it comes in contact, and the curvature of the free mercury surfaces. 

 This curvature depends on the surface tension of the mercury surface 

 T and the pressure difference Ap between the inside and outside of the 

 mercury at the point under consideration. That is, 



where Ri and R2 are principal radii of curvature of the surface (radii 

 taken in planes cutting the surface at right angles to each other). 



In this switch the pressure difference is a function of the height of the 

 point under consideration above the surface of the reservoir: 



Ap = pgh. (2) 



Substituting the values 



p =13.6 g/cc for mercury, 



g = 980 cm/sec^ and 



T = 450 dynes/cm for mercury, 



