BALANCED POLAR MERCURY CONTACT RELAY 1411 



ADJUSTMENT OF SENSITIVITY 



The procedure used to adjust the relay to any desired pair of sensi- 

 tivity values is as follows: 



L Magnetize the two magnets fully by a flux directly across them. 



2. With the armature starting from one side, progressively decrease 

 the magnetization on that side in small steps until the armature just 

 operates to the other side on the final current value desired for this iluw- 

 tion of operation. 



3. With the armature starting on the other side, progressively decrease 

 the magnetization on that side in small steps until the armature just op- 

 erates from that side on the final current value desired for this second 

 direction of operation. 



4. Repeat 2 and 3 alternately until the relay just operates in both 

 directions on the two current values desired. 



Within limitations such as the size of demagnetizing increments used 

 per step, this procedure results in an adjustment to the two test values 

 used. Let us consider why this result is obtained. 



If we should substitute p = 1 in equations 9 and 10 we would obtain 



Mc, = M, — Mo, 

 Men = M, - Mn . 



That is, in such a case, the two adjustments would be independent of 

 each other and the above process would require only one adjustment of 

 each magnet. In the more general case, where the force on the armature 

 is affected by both poles, each adjustment on one side results in a greater 

 magnet strength at the pole being adjusted than is required for the final 

 adjustment, because of the pull from the opposite pole, the amount of 

 which is greater than normal because that pole has not been reduced to 

 its final magnet strength. Thus the final adjustment is normally reached 

 through a number of alternations between the two sides, which progres- 

 sively approach the final pair of sensitivity values. 



The sequence of sensitivity values obtained in a typical adjustment of 

 this kind is shown in Fig. 13. 



