BALANCED POLAR MEHCLKY ('()NT.\( r ICKLAY 1399 



When the ball falls out as the result of an operation, the loss of mer- 

 cury results m a small, temporary constriction of the fillets in the grooves 

 near the contact that was just opened. That is, the curvature of the 

 surfaces of these fillets becomes more negative. As shown by equation 

 (1), this produces a local decrease in liquid pressure. Mercury therefore 

 flows up the armature from the reservoir to restore the normal static 

 balance between surface tension and negative head. This is the fun- 

 damental behavior of an ordinary wick. The ball drops into the reser- 

 voir unless it happens to hit the armature on the way down. Thus, for 

 repeated operations, there is a continuous circulation of mercury. 



As indicated in Fig. 3, the fixed contact is a ball, with a flat surface 

 where contact is made to the armature. In Fig. 4(c), taken directly after 

 the breaking of the mercury bridge, the mercury remaining at the fixed 

 contact on the right has been thrown back on the contact by surface 

 tension forces, laying bare the flat surface. After several flow oscillations 

 that are not shown, it comes to rest with the spherical contour shown 

 by the contact at the left in Fig. 4(a). That is, being disconnected from 

 the reservoir and having a limited wet surface to spread over, it assumes 

 a positive head corresponding to a positive spherical radius about equal 

 to that of the contact. This provides a mercury ''cushion" in the form 

 of a segment of a sphere, to which contact is made when the relay 

 operates. 



ADJUSTMENT OF SENSITIVITY 



For various combinations of MMF's (magnetomotive forces) in the 

 two magnets, various corresponding pairs of sensitivity values exist for 

 operation in the two directions. A theoretical analysis of this relationship 

 is given in the attached appendix. The adjustment of magnet strengths 

 to obtain a specified pair of sensitivity values is made on the completed 

 relay, using two electromagnets placed outside the can opposite the 

 relay magnets. An automatic circuit is provided for this purpose that 

 makes a complete adjustment in about 15 seconds, the time being de- 

 pendent upon the precision required and the uniformity of the product 

 being adjusted. The procedure used and the basis for it are discussed in 

 the Appendix. 



All of the adjustments used are of the type for which the armature 

 moves all the way from one contact to the other when an operate? current 

 is applied. This represents the condition for the minimum differential 

 ampere turns obtainable between the two operate values l)ecause it 

 makes use of armature momentum. 



