488 



ELECTRICAL MEASUREMENTS 



Sangamo ampere-hour meter has been developed. In this instru- 

 ment the driving field is produced by a large permanent magnet. 

 In electric automobile and truck work, an ampere-hour 

 meter will give an indication of the state of charge of the battery. 



FIG. 280. Sangamo ampere-hour meter. 



The Mercury Watt-hour Meter. The electrical connections for 

 the Sangamo direct- current watt-hour meter are shown in Fig. 

 281 A. The main-line current passes across the copper armature 

 disc in the direction EiE*. The U-shaped electromagnet 7, 

 which furnishes the necessary field, is connected across the line. 



The light-load adjustment is obtained by the use of a thermo- 

 couple H which is inserted in a shunt circuit between EI and E^ 

 and heated by a resistance coil which forms a part of the potential 

 circuit of the meter. The couple sends a small current through 

 the disc in the same direction as the load current. The effect of 

 the thermo-couple is controlled by altering the position of the 

 connecting link K. The couples are now made reversible, so 

 that the same meter may be used in either the positive or nega- 

 tive side of the line. 



As the fluid friction naturally becomes unduly large with in- 

 crease of armature speed, the instrument is compounded by taking 

 the main circuit around the U-magnet at CT. This improves 

 the action of the instrument at heavy load and at overload. 



