Harrison-Foote compensated indicator

Thus when both instruments read correctly for an external resistance of 2 ohms if the external resistance is increased by an ohm, the indicator of low resistance is in error by 7.7 per cent or about 77°C. at 1,000°C., while the instrument of high resistance still reads practically correct. Corrections such as the above emphasize the importance of using a galvanometer having a resistance of 300 ohms or more. In actual operation the line resistance may change by several ohms on account of bad contacts and the deterioration of the thermocouple. The error due to variable line resistance is eliminated in the Harrison-Foote compensated indicator described later, even though the resistance of the indicator is low. 

Harrison-Foote Compensated Indicator.—This instrument manufactured by the Brown Instrument Co. is illustrated in simple form by Fig. 5. The circuit CDGF is an ordinary millivoltmeter in which G represents the moving coil. 

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