COMPARISON OF GAS AND PLATINUM SCALES. δ = 1·5. ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┬▬▬▬▬▬▬▬▬▬▬▬▬ Platinum Thermometer│Air Thermometer Reading│ Difference Reading (Pt.). │ _t_ (deg. C.). │(_t_ ▬ Pt.). ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┼▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┼▬▬▬▬▬▬▬▬▬▬▬▬ -100 │ -97·1 │ +2·9 │ │ 0 │ 0 │ 0 50 │ 49·6 │ -0·04 100 │ 100 │ 0 200 │ 203·1 │ 3·1 300 │ 309·8 │ 9·8 │ │ 400 │ 420·2 │ 20·2 500 │ 534·9 │ 34·9 600 │ 654·4 │ 54·4 700 │ 779·4 │ 79·4 800 │ 910·7 │ 110·7 │ │ 900 │ 1049·4 │ 149·4 1000 │ 1197·0 │ 197·0 1100 │ 1355·0 │ 255·0 1200 │ 1526·7 │ 326·7 1300 │ 1716·0 │ 416·0 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┴▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬┴▬▬▬▬▬▬▬▬▬▬▬▬ =Terms used in Resistance Pyrometry.=—Following on the researches of Callendar and others, certain terms relating to resistance pyrometers have come into use, and will now be defined. (1) _The Fundamental Interval_ is the increase in resistance between 0° C. and 100° C, or R_{100}-R_{0}. It should be remembered that the increases between 200° and 300°, or 800° and 900°, all temperatures being taken on the gas scale, differ from the fundamental increase. (2) _The Fundamental Coefficient_ is that fraction of the resistance at 0° C. by which it increases per degree between 0° and 100°, on the average, or (R_{100} - R_{0}) ───────────────── . (R_{0} × 100) This figure is in reality the average temperature coefficient between 0° and 100°. For pure platinum the value is or 1/260 or 0·003846. (3) _The Fundamental Zero_ is the temperature, on the platinum scale, at which the resistance would vanish; it is evidently the reciprocal of (2), prefaced by a minus sign, or (R_{0} × 100) ─ ───────────────── . (R_{100} - R_{0}) For pure platinum this temperature would be -260_p_, since it is assumed that the average increase or decrease per degree holds throughout; that is, for every degree the metal is cooled the loss of resistance is taken to be 1/260 the resistance at 0°. Hence at -260_p_ the resistance, on this assumption, would vanish.