to face page 140. 



Fio. 40. Meicury Switch, top removed. At left, the switch with its mercury cups and comparison 

 coils ; at right, the top with the keys and copper links. In the foreground are shown the com- 

 parison coils and nuts for holding top in place. 



FIG. 4t. General View_of Mercury Switch. 



Fio. 42. Under side of Mercury Switch, showing electrical connections. 



