M2 (1915 to 1926) Previous
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The M2 meter was of a new design and easier to work with than the M and M1 meters. The enclosure and internal frame were now made of cast iron instead of punched steel. The potential coil on this meter was the same as used on the previous M1 meter, the coil having 2 sections connected in series on a U-shaped core. The light load adjustment was changed to a screw-type adjuster that allowed for more precise adjustment. The M2 used the same lag-load adjustment plate as the M1. The M2 came in three sizes, depending on capacity: 5-25A, 50A-75A, and 100-400A. The mid-size style exists in 2 versions - the earlier one shown below and a later style where the sides taper outward an inch towards the bottom.
 
M2, 5-25A
David Turner collection
M2, early 50A
Handbook for Electrical Metermen
M2, 100A
Watertown Municipal Utilities collection