=Standardization by Fixed Points.=—Taking any millivoltmeter which, with a maximum of 20 millivolts at the terminals, will give a full scale deflection, the first step is to arrange that the pointer (or spot of light) shall just remain on the scale at the highest temperature to be attained by the junction. This may be done by placing the hot junction in boiling water and noting the deflection obtained, either in millivolts or equal arbitrary divisions, and also the temperature of the cold junction. The deflection observed is due to a difference of temperature (100-_t_) deg. C, where _t_ is the temperature of the cold junction. If the highest temperature to be measured is 10 times (100-_t_), the deflection should be rather less than 1/10 of the scale, and similarly for any other required temperature limit. If the observed deflection exceed this proportion, a series resistance should be added until the correct value is obtained. This resistance is then permanently installed in the circuit for use with the junction under trial. Before proceeding further it is necessary to consider whether the pyrometer is to possess a single cold junction of ascertainable temperature (as in fig. 6), or whether it will be arranged with two cold junctions in the head, as in fig. 4. In the former case it is simpler to prepare a “difference” scale; that is, one which reads differences of temperature between the hot and cold junctions, from which the temperature of the hot end may be obtained by adding to the difference that of the cold junction. In the latter case the cold end should be kept by artificial means at the temperature likely to be attained in practice—say 25° C.—a water-bath being suitable for this purpose. It is advisable to remove the shield of the pyrometer when standardizing, so as to expose the hot junction, as closer readings can then be taken. A number of materials—preferably cheap—of known boiling points or melting points are then selected from a table of fixed points (page 16) so as to give about six points, distributed fairly evenly over the scale. As an example, if it were desired to prepare a temperature scale from 0° to 1000° C., the following might be chosen:— ════════════════════════════════════════════════╤═══════════════════ Substance and Condition. │ Temperature. ────────────────────────────────────────────────┼─────────┬───────── Water at boiling point │ 100° C. │ 212° F. Tin at melting point │ 232 │ 449 Zinc at melting point │ 419 │ 786 Antimony at melting point │ 631 │ 1167 Common salt at melting point │ 800 │ 1472 Copper at melting point (covered with graphite) │1084 │ 1983 ════════════════════════════════════════════════╧═════════╧═════════