Prof. H. L. Callendar on Platinum Thermometry. 197 



Ihe present time are required to be typewritten. It is for 

 this reason desirable to avoid, wherever possible, the use of 

 Greek letters and subscript diacritics and indices. 



The Fundamental Interval. — The denominator, B/ — R°, in 

 formula (1) for the platinum temperature pt, represents the 

 change of resistance of the thermometer between 0° and 

 100° C, and is called the fundamental interval of the thermo- 

 meter, in accordance with ordinary usage. It is convenient, 

 as suggested in a previous communication, to adjust the 

 resistance of each thermometer, and to measure it in terms 

 of a unit such that the fundamental interval is approxi- 

 mately 100. The reading of the instrument will then give 

 directly the value of pt at any temperature, subject only to a 

 small percentage correction for the error of adjustment of the 

 fundamental interval. 



The Fundamental Coefficient. — The mean value of the tem- 

 perature-coefficient of the change of resistance between 0° 

 and 100° C. is called the fundamental coefficient of the wire, 

 and is denoted by the letter c. The value of c is given by the 

 expression (B/ - R°)/100 R°. The value of this coefficient is 

 not necessary for calculating or reducing platinum tempe- 

 ratures, but it is useful for identifying the wire and as giving 

 an indication of its probable purity. 



The Fundamental Zero. — The reciprocal of the fundamental 

 coefficient c is called the fundamental zero of the scale of the 

 thermometer, and. is denoted by the symbol pt°, so that 

 pt°=l/c. The fundamental zero, taken with the negative 

 sign, represents the temperature on the scale of the instrument 

 itself at which its resistance would vanish. It does not 

 necessarily possess any physical meaning, but it is often more 

 convenient to use than the fundamental coefficient (e. g. y 

 Phil. Trans. A, 1887, p. 225). It may be remarked that, 

 if the resistance has been accurately adjusted so that the 

 fundamental interval is 100 units, R°, the resistance at 0° C, 

 will be numerically equal to pt°. 



The Difference Formula. — It is convenient to write the 

 formula for the difference between t and pt in the form already 

 given (2), as the product of three factors, d x (£/100— -1) x t/\ 00, 

 rather than in the form involving the square of £/100, which I 

 originally gave, and which has always been quoted. Owing 

 to the form in which it was originally cast, I find that most 

 observers have acquired the habit of working the formula in 

 the following manner. First find the square of f/100, 

 then subtract £/100, writing the figures down on paper, and 

 finally multiply the difference by the difference-coefficient d 

 with the aid of a slide-rule. It is very much easier to work 

 Phil. Mag. S. 5. Vol. 47. No. 285. Feb. 1899. P 



