ABSTRACTS 



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APPARATUS. — Potentiometers for thermoelement work. Walter P. 

 White. Bull. Amer. Inst. Min. Met. Bng. 1 763-1 772. Sept., 

 1919. 

 Thermocouple pyrometers are read in three ways. First, by direct 

 readers where the current, and therefore the deflection, is proportional 

 to the electromotive force of the couple; second, by potentiometers 

 where the galvanometer merely helps to balance the electromotive 

 force of the couple against that of a standard cell by means of known 

 resistances and a constant battery-current; third, by intermediate 

 instruments such as the pyrovolter, employing the potentiometer prin- 

 ciple with a constant battery, but avoiding the standard cell, and 

 measuring current with a calibrated galvanometer. Similar in result, 

 but different in principle, is the new Harrison-Foote instrument, where 

 the circuit resistance can be very quickly adjusted to the correct value. 

 All these special instruments avoid the main difficulty of a direct 

 reader, namely, the error from uncertain or variable resistance. It is 

 necessary to use the regular potentiometer in order to avoid also the 

 uncertainty (perhaps i per mille) of the calibration of the direct reader. 

 With a slide-wire a simple and portable potentiometer is made, good 

 to about 10 microvolts, or 0.25° with most thermocouples. The slide- 

 wire also permits readings to i microvolt, though not altogether satis- 

 factorily. Two special designs of potentiometer, the Diesselhorst- 

 WoLff and the White, enable readings to be made to o.i microvolt or 

 better, and the White potentiometer is very little affected by corrosive 

 gases. Both these are deflection potentiometers, enabling part of the 

 readings to be taken direct from the galvanometer with a gain in speed 

 and without sensible error. If the potentiometer is arranged as a 



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