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To calibrate a platinum couple, the most convenient fixed points are 
the fusion temperature of copper, antimony and zine. 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 uniformly heated. 
Another form of furnace which is less likely to get out of order, but 
which on the other hand is not so satisfactory for precise work, is shown 
in Fig. 8. This consists of a rectangular trough of brick work (a, Fig. 8). 
Cc Cc 
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 ce, which 
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. 
