Ans. They may become weak with age, or affected by the proximity of other magnetic fields. The series coil of the meter may, under short circuit so affect the strength of the permanent magnets as to render the meter inaccurate.
Ques. What precautions are taken to keep the strength of the permanent magnets constant?
Ans. Weakening with age is prevented by the process of "Aging." The effect of neighboring fields is overcome by iron shields; this prevents the electromagnets affecting, through overloads, the strength of the permanent magnets.
4. The Registering Element.—This mechanism comprises the dials, pointers, and gear train necessary to secure the required reduction in speed. This gear train is driven directly by the rotor and therefore its friction should be low and constant. The dials should be easily read and should register directly in kilowatt hours. If a constant be used to reduce the reading to kilowatt hours, it should be some multiple of 10, to avoid errors in multiplication. By means of suitable gears in the meters this is easily accomplished.
Fig. 2,549.—Register dial of Sangamo single phase induction watt hour meter (full size). The dial circles read 10, 100, 1,000, and 10,000 kilowatt hours from right to left.
5. The Mounting Frame and Bearings.—These parts have an important influence on the accuracy of the meter, as it is in the bearings that most of the friction in the meter occurs. The frame should be rigid and free from vibration, so that the bearings will be at all times in perfect alignment.
Initial friction is unavoidable in any meter construction and can be easily compensated for. A change in the initial friction, however, due to wear of bearings, makes readjustment necessary.
In selecting a meter the special attention should therefore be given, to the construction of the bearings, particularly the lower, or "step" bearing which supports the weight of the moving element.