354 



DR. J. A. HARKER ON THE HIGH-TEMPERATURE STANDARDS 



a plan of the top of the instrument in fig. 5. It is designed with a view of rendering 

 possible the use of a short slide-wire of large cross section. The balancing coils, on 

 which the fall of potential is adjusted to a definite value, are in two rows, the centre 



Fig. 5. Plan of potentiometer. 



row of the box consisting of 20 coils of yg of an ohm each. In series with 

 these is a second row, immediately behind the bridge-wire, consisting of 11 coils 

 of y^y of an ohm each. By means of an arrangement of thick copper bars 

 connected with the ends of the slide-wire, which has a total resistance of "02 ohm, 

 any two adjacent coils of this latter series may be put in parallel with the slide-wire. 

 The 11 coils of '01, two of which are thus shunted, are therefore always exactly 

 equivalent to '] ohm. 



For ordinary thermoelectric work the fall of potential along these two sets of coils 

 is adjusted so that each of the back row represents 1000 microvolts, each coil of 

 01 ohm being therefore 100 microvolts. The slide-wire, 200 millims. long, is 

 provided with a divided scale on which Y5W5 P ar ^ f its length can be easily 

 estimated. It will be seen that the slide-wire thus connected acts like a vernier to 

 the small coils. 



The adjustment of the electromotive force is made by a standard Clark or Weston 

 cell and the auxiliary set of coils in the back row, a feature of the instrument being 

 that without any external alteration either form of standard may be used at will. 

 The five coils to the left are permanently connected in series, but are arranged so 

 that any coil may be cut out of circuit when required. Their values are 100, 40, 1, 

 5, and '5 ohms respectively. Those to the right are a set of 10 simple series coils 

 of '01 ohm each, arranged so that a connexion can be taken from any one of them to 



