ELECTRICITY METERS 



487 



projecting downward. The clearance of the spindle in the tube 

 is about 0.006 in. and the form of the chamber is such that the 

 mercury cannot be spilled even though the instrument be 

 inverted. 



As the current flows diametrically across the disc, the flux must 

 be directed upward on one side of the spindle and downward on 

 the other side. The driving field is furnished by either a perma- 

 nent or an electro-magnet, according to circumstances; the 



Upper Bearing 

 Screw 



Arch. 



Rotating Disc Submerged 

 in Mercury 

 (Armature) 



Lower Jewel Guide 

 Magnet Holding Ear Pole Plate 



FIG. 279. Section of working parts of Sangamo meter. 



poles are immediately beneath the chamber and contiguous to 

 the current lugs. The magnetic circuit is completed by the 

 spirally laminated soft iron return plate. The necessary " brak- 

 ing" action is due to induced currents in the armature disc and 

 in the usual aluminum damping disc provided for that purpose. 

 The Sangamo Ampere-hour Meter. Aside from the ordinary 

 lighting and power installations, there are certain operations, 

 such as electroplating and the charging and discharging of 

 storage batteries, where it is desirable to register the total quan- 

 tity of electricity rather than the energy. For this purpose, the 



