154 THE ROYAL SOCIETY OF CANADA 



This value of a is called the temperature coefficient. Substituting 

 it in the first formula, we obtain the following expression for the tempera- 

 ture in terms of resistance. 



100 (Rt-Rp) 

 t — 



R 100 Ro 

 Since any calculation of temperature involves the quantity (Riog — R^), 

 this factor is called the fundamental interval or F.I. 



The temperature indicated by a resistance thermometer will be 

 the true temperature, if the change in resistance follows a linear law. 

 Hence the indicated temperature must undergo a correction, the 

 amount of which depends on the constants in the equation expressing 

 the above law, and on the temperature. In the case of a platinum 

 thermometer, Callendar found that the correction would be given by — 



f' y 100 _/ 100 



T=true temperature. 



tp = platinum temperature. 



5 = a constant depending on the quality of the wire. 



To obtain the d, the thermometer must be accurately observed 

 at three different temperatures. Since 0° C is given by melting ice, 

 and 100° C by steam under standard pressure, these are adopted as 

 standard temperatures, the comparison temperature usually being the 

 boiling point of sulphur. 



A formula, analogous to the one above is assumed to hold for iron. 

 The value of the d is then obtained as follows. 



The resistances of an iron and a platinum thermometer are obtained 

 when at the same temperature. The true temperature is calculated 

 from the platinum reading by the formula above, and is used in a similar 

 formula in conjunction with the iron indication, thus giving a value 

 of the d for iron, when that particular comparison temperature is used. 

 This scheme is carried out at different temperatures and the values of 

 the Ô are then plotted. The behaviour of the iron wire is recorded 

 in this way, for temperatures between O'' and 100° at intervals of 10°. 



The apparatus is arranged on the Wheatstone's bridge principle, 

 and consists of a battery B, an external resistance R, two equal ratio 

 resistances P and Q, an adjustable arm composed of two resistance 

 boxes, and the thermometer, the last two arms being joined by a bridge 

 wire. The galvanometer is connected to the middle point of the two 

 ratio coils and to a movable contact on the bridge wire. 



The ratio arms each consist of three ohms of manganin wire, and 

 are adjusted to equality. Since the ratio is unity, all connecting wires 

 on opposite sides of the galvanometer contact are made equal in resist- 



i 



