BALANCED POLAR MERCURY CONTACT RELAY MOO 



Also shown on the figure is a typical spring characteristic 



fml ti 



(4) 



If the stops li and A are placed at points of intersect iofi of the spring 

 characteristic with the magnetic characteristic for M = 0, (in this case 

 dXlJli = 0.48), we have a condition that corresponds to a hypothetical 

 relay of infinite sensitivity. That is, the armature can be released from 

 contact with either side with an infinitesmal coil input, and, if there are 

 no magnetic or mechanical losses in its travel, it \vi\\ just swing to the 

 opposite side without any change in the total energy of the system. I>et 

 us define the magnet strength for this condition as Mo and let us assume 

 that Mc = for operation in either direction with this magnet strength. 



Assume now that the magnet strengths on each side are increased 

 equally to the values 



Mni = Mn = Mo -h A. (5) 



For values of A/A near 1 this change can be balanced out by a coil input 

 of about the same amount, as practically all of the pull on the armaimc 

 would be from the nearer pole. For values of /i/A near the effect of a 

 coil input will be equal and aiding in both gaps. The coil inputs required 

 to just operate from the N and *S poles, defined as Mcn\ and Mc,\ , re- 

 spectively for this particular case, will be 



Mcsi = - Mcni = pA = piMni - 3fo) = p(M.i - 3/o), (6) 



where p is a value between 1 and 0.5. Values for individual cases can Ix; 

 worked out with reference to the curves for various values of MJMn in 

 Fig. 12. For the case shown, where (Jti = 0.48, p is about 0.8. 



An adjustment in accordance with (6) is thus a balanced one with a 

 spread of 2pA between the two sensitivity values, the amount of spread 

 increasing with increase in the strength of the two equal magnets M^i 

 and Msi above the value Mo . 



A general type of adjustment, including all possible combinations of 

 the two sensitivity values, can be obtained by adding a suitable value B 

 to a balanced pair of sensitivity values in accordance with equation (6), 

 These general sensitivity values would then be 



Mc, = Me,l + B, /yx 



Men = Mcnl + B. 



This is the type of change that would be produced by a bias oi -B 



