Divect-readimi Fotentiometer fo7' Thermoelectric Work. 41 



This example is sufficient to indicate the importance of 

 choosing the resistances suitably. 



At the same time it should be remarked that in several 

 ways the low-resistance potentiometer is less easy to construct 

 and work ; for instance, it may require much larger steady 

 currents ; and these practical considerations may sometimes 

 outweigh the advantages o£ greater sensitivity. 



y. A Direct-readinq Potentiometer for Thermoelectric Worl'. 

 By J. A. Harker, D.Sc* 



[Plate II.] 



TO anyone wdio has used a potentiometer for thermoelectric 

 measurement of temperature, it will be obvious that 

 nearly all the ordinary types of instrument have material 

 disadvantages when employed for this purpose. 



Most of the higher class potentiometers are primarily 

 designed for direct comparison of E.M.F.'s of the order of a 

 volt and upwards, the readings being made in most cases to 

 about '0001 volt. With the thermojunction materials now 

 employed for high-temperature measurement, this quantity 

 100 microvolts represents 6° to 10° G. ; and in order to 

 attain higher sensitivities at low ranges, special accessor}^ appa- 

 ratus is required to reduce the standard E.M.F. applied to the 

 potentiometer to ^^j or ywo ^^ ^^^ normal value. This addition 

 is generally accompanied by a considerable loss of propor- 

 tionate sensitivity. 



It is obvious that in thermoelectric work the circuit in 

 which the E.M.F. to be measured is generated has generally 

 a total resistance of quite a different order of magnitude to 

 the cases most usually met with ; and so far from a high 

 resistance in the potentiometer being an advantage as in many 

 cases, it is in this case detrimental to the attainment of a high 

 sensitiveness. 



The diameter and length of the wire used for thermo- 

 junctions of course varies greatly; but we shall be within the 

 mark in saying that the total resistance of the thermojunction 

 wire seldom exceeds 5 ohms, and that therefore a low resist- 

 ance in the potentiometer is a great advantage. In the form 

 to be described^ intended for the direct measurement of 

 E.M.F.''s up to 21,000 microvolts, the maximum resistance 

 capable of being placed in series with the galvanometer and 

 thermojunction is 2*1 ohms. 



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



