OF THE NATIONAL PHYSICAL LABORATORY. 355 



the long copper bar just in front. Suppose we wish to use as a standard a Clark 

 cell whose electromotive force at the prevailing temperature is T4333 volts. It is 

 obvious that we may make the fall of potential over the "1 ohm balancing coils have 

 the desired value of 1000 microvolts by putting into circuit coils 100, 40, 1, -5, and 

 three of the "01 series, leading from the third hundredth by means of the copper bar 

 to No. 17 of the balancing set, when altogether we shall have 



100 

 40 



1 [> in the back row, 

 5 



03 J 

 17 



L7 "I 



> in balancing coils and bridge- wire, 



making in all 143'33 ohms. 



Should a Weston cell having an electromotive force of 1'OISG volt be substituted 

 for the Clark, the only alteration necessary would be to short-circuit coils 40, 1 , and 

 5, and to move the connector from the third to the sixth of the set of hundredths. 

 The compensating current is furnished by a small secondary cell, in series with which 

 is a dial resistance capable of variation up to 200 ohms by steps of '005 ohm. 



The four thermojunction circuits provided are connected to a selector switch, by 

 means of which each successively or any two of them connected in opposition may be 

 brought into circuit, and the change-over from the standard cell connexion required 

 in the preliminary adjustment is made at the two-way switch at the front left- hand 

 corner, by means of which the galvanometer may be put into the circuit desired. 



Constructional details are given in the paper referred to. All the coils employed 

 are of selected manganin, carefully annealed, and all connexions are made by mercury 

 caps and copper short-circuiting pieces, the only metals employed anywhere in the 

 parts carrying current being copper and manganin. 



The values of those coils in the box which were used in this work as determined at 

 the conclusion of the comparison experiments are given in the following table. Any 

 alteration in their relative values which had taken place since the first standardiza- 

 tion is undoubtedly so small as to be quite negligible compared with other errors in 

 thermocouple work.* 



* It is obvious that in building clown to obtain a convenient standard of thermal electromotive force 

 100 or 1000 microvolts so long as the relative values of the coils employed remain the same, their absolute 

 value is of no moment. 



A matter of great importance, however, is to measure the value of each coil in exactly the same way as 

 it is used. Accordingly, for this standardization a potential method of measurement was employed in all 

 pases, the current and potential leads being connected exactly as in actual work. 



2 z 2 



