486 ELECTRICAL MEASUREMENTS 



The current enters at the + terminal Ci, flows through the 

 mercury to the amalgamated edge of the copper disc CD, then 

 through the disc to its central portion, which is amalgamated, and 

 out by the terminal C 2 . Thus the current in the disc is in the 

 field of the permanent magnets SD and a driving torque is imparted 

 to the disc armature. To protect the copper from the action of 

 the mercury the top and bottom surfaces of the disc are platinum- 

 plated and enameled, except directly above C 2 . As the arma- 

 ture moves through the fields of the two magnets SB and SD, 

 there will be the usual braking action due to eddy currents. The 

 fluid friction of the mercury also contributes a retarding action and 

 as this increases with the speed, that is, with the customer's load, 

 the meter is compounded by a coil of a few turns, CC, on the 

 lower iron crossbar. When the current is increased, the 

 strength of the field SD is also increased and hence the driving 

 torque becomes larger. However, the action of the magnetic 

 brake remains the same, for the poles at SD are so arranged 

 that when the field at SD is increased that at SB is diminished. 



The buoyancy of the armature is adjusted by a weight on the 

 spindle until the disc just sinks. Friction between the pivot and 

 jewel is thus reduced to a minimum. A sealing device is used so 

 that the mercury will not be spilled during transportation. 



Sangamo Meter. In America the mercury motor meter has 

 been developed by the Sangamo Electric Co. which began the 

 work in 1904. 



> The main body of the mercury chamber is made of a moulded 

 insulating compound (see Fig. 279) . The two current terminals, 

 EI, Ez, are diametrically opposite each other, and above the lower 

 part of the chamber which contains the copper disc armature 

 is a spirally laminated ring of soft iron (return plate). On 

 the spindle above the disc is a hardwood float; this takes the 

 pressure from the lower bearing, which becomes merely a guide; 

 in fact, a slight thrust is exerted against the bearing plate of the 

 upper ring jewel. 



In all Sangamo meters the copper armature discs are now slit 

 radially. The current is thus caused to flow directly from ter- 

 minal EI to E 2 without spreading over the disc. By this means 

 the torque is increased about 40 per cent. 



The cover of the mercury chamber is made with a central tube 



