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To calibrate a platinum couple, the most convenient fixed points are 

 tlie fusion temperature of copper, antimony and zinc. These metals may 

 be melted in small graphite crucibles. The size of crucible chosen and the 

 quantity of metal used should be such that at least 5 cm. of the couple may 

 be immersed in the metal. The crucible may be heated in any suitable 

 manner, but an electric resistance furnace is perhaps the most convenient. 

 Fig. 7. 



One form of furnace consists of two concentric cylinders of fire clay, 

 or porcelain, placed upon a base of the same material. A suitable cover 

 also is provided with a hole for admitting the couple. The inner cylinder 

 is overwound with fine nickel wire or ribbon and the crucible, to be heated, 

 is placed within this cylinder. It should be placed at about the center 

 so as to be uniformlj' heated. 



Another form of furnace which is less likely to get out of order, but 

 which on the other hand is not so satisfactorj- for precise work, is shown 

 in Fig. 8. This consists of a rectangular trough of brick work (a, Fig. 8). 



c c 



Fig. S. 



The inside width should be somewhat greater than the diameter of the 

 crucible to be used and the depth slightly greater than the height of the 

 crucible. The ends of the trough are closed with carbon plates cc, v/hich 

 carry binding posts bb, for the connecting wires. The intervening space 

 K is filled with a granular resisting material, commercially known as 

 "Kryptal." The connectors at bb are connected to some source of e.m.f. 

 either DC or AC. The amount of current may be regulated by varying the 

 density of the mass of kryptal used. Thus, when a large amount is used 

 and when it is packed down well a large current will pass through the 

 furnace. The top of the entire furnace should be covered over with 

 bricks. 



