| Platinum | Air Thermometer | Difference |
| Thermometer | Reading | (t - Pt.). |
| Reading (Pt.). | t (deg. C.). | |
| -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 R100-R0. 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
| R100 - R0 |
| ————— |
| R0 × 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
| R0 × 100 |
| - ————— |
| R100 - R0 |
For pure platinum this temperature would be -260p, 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 -260p the resistance, on this assumption, would vanish.
(4) The Difference Formula is the expression which gives the relation between gas-scale and platinum-scale temperatures, or