Shallenberger Integrating Wattmeter (1894 to 1897) Previous
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By the mid-1890s, Shallenberger's ampere-hour meter was popular but because of the increasing use of motors, a true watthour meter was needed to account for varying voltages and power factors. Shallenberger rose to the challenge and came up with a new meter which was the first commercially produced induction watthour meter. It was large, heavy (41 pounds!), and more than twice as expensive as comparable meters in its time. This meter was one of the first models to use a cyclometer register. Depending on the customer's preference, this register was equipped with 4 drums (registering in kwh) or 7 drums (registering in watthours). The stator was similar to ones in later meters with its voltage and current coils arranged on opposite sides of the disk and had a magnet assembly to damp the disk's speed.
 
Shallenberger Integrating Wattmeter
Westinghouse Handbook on Watthour Meters
Meter internals
Westinghouse Handbook on Watthour Meters