METHODS AND APPARATUS. 



23 



desired value was approximately obtained. A careful determination was then 

 made by tying the Beckmann and Richter thermometers together and immers- 

 ing the pair in a Dewar flask similar to that used in the constant-temperature 

 oven. The flask was filled with warm water stirred by compressed air, and 

 immersed in a large body of hot water in an outside pail, the temperature of 

 this outside water being regulated by an electric-coil heater. This heater is 

 simply a resistance wire inclosed in a water-tight spiral tube, having in its 

 outside electrical circuit a variable resistance, so that the amount of heating 

 can be controlled. Several simultaneous readings of the thermometers were 

 taken, as shown by the data given in table 1 which represent the determina- 

 tion of the zero point of Beckmann thermometer 40724, used in the constant- 

 temperature oven. The Beckmann reading, corrected for caliber errors, must 

 be multiplied by 1.008 the constant determined from the above calibra- 

 tion and subtracted from the true temperature in order to give the zero 

 point of the thermometer. 



Table 1. Determination of zero point, Beckmann thermometer PTR 40724. 



Thus the correct temperature as determined from a reading of this ther- 

 mometer is given by the formula 37.300+1. 008 X reading. When working only 

 to hundredths, the formula 37.30+1.01 Xreacling gives closer results at the 

 lower part of the scale, but 37.29 + 1.01 Xreading is better on higher readings. 



METHOD OF OPERATING APPARATUS. 



In measuring temperatures with this apparatus the procedure is first to 

 connect the partly discharged cell to the measuring circuit for a few minutes 

 in order to let the current become constant, and then carefully to depress the 

 key Gi. In general, a galvanometer deflection results, indicating that the 

 current differs from its standard value. By regulating the variables V 3 and 

 V 4 , and also, if necessary, Vi and V 2 , this deflection is brought as nearly as 

 possible to zero, thus indicating that the current has closely enough its proper 

 value. The current having been adjusted, the next step is to close G 2 and 

 one of the switches 1, 2, 3, 4, etc., according to which thermal junction is to be 

 used. In general a deflection results, which can be almost balanced by 

 properly moving the potentiometer contacts. The attempt is not made to 



