Type C (1904, 1905) Up
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After being barred from making induction meters until the end of 1910 (following a patent lawsuit brought by Westinghouse) in 1903, Robert Lanphier sought out a design for a meter that didn't infringe on Tesla's patents (held by Westinghouse). He decided to work with the mercury-motor meter which was widely used in England at the time, and developed the first practical mercury-motor watthour meter. The design of this meter was unique, since this meter used the voltage coil to generate the braking field instead of the permanent magnets normally used for the brake. The coarse full-load adjustment was made by adjusting the resistance of a shunt inside the meter, and the fine adjustment was made by adjusting the depth of the contacts into the mercury chamber. The light load adjustment was made by varying the resistance of the voltage circuit. Unlike the later mercury-motor meters, this model depended on the disk's natural buoyancy in mercury alone. The rotation of the element was indicated by a hand on the top of the shaft assembly and was visible through a window set in top of the meter cover (which was factory-sealed just like the previous Gutmann Type B meter). This design had several major flaws, the fatal one being that the material used for the mercury chamber softened when the temperature of the surroundings got too warm and leaked. All the Type C meters were recalled and replaced with an improved version (the first version of the Type D). Type C
Electrical Measuring Instruments and Meters