C>GS 



Mr. I\. A. Lehfeldt on a Potentiometer 



approximation and the remaining three trials to get a second 

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could be gained by this extra labour, and this illustration of 

 the circle fully suffices to indicate the comparative ease with 

 which the new method may be used on a hitherto unsolved 

 type of problem. The labour would not have been much 

 greater had we required a circle (or any other three-constant 

 curve) through even a dozen points. 



LXX. A Potentiometer for Thermocouple Measurements. 

 By R. A. Lehfeldt*. 



rpO make a satisfactory potentiometer for thermoelectric 

 J- work, it is essential that it shall not introduce a high 

 resistance in the circuit of the couple and galvanometer. Most 

 of the potentiometers in the market, which answer well enough 

 for comparing voltaic cells, fail in this respect. I have there- 

 fore devised an instrument which is shown schematically in 

 fig. 1. From the positive terminal of the accumulator B 

 current flows to the switch K by means of which it can be 



* Communicated hv the Physical Society : read March 13, 1903. 



