SPECIFIC HEAT OF AMMONIA. 333 



2. Energy Measuring Apparatus. 



a. Circuits. Figure 4. 



Arranged to measure current through heating coil, and 

 voltage drop across heating coil, alternately, by potentio- 

 meter method. The " dummy " heater, equal in resistance 

 to the heating coil, keeps the line hot and the batteries at 

 the desired voltage while the heater switch is off. 



b. Heating Coil. 



Resistance 12.2 ohms. Manganin wire, laced vertically 

 on thin baked mica strip, ^ enclosed by two baked mica 

 cover strips. Manganin wire used on account of its zero 

 temperature coefficient resulting in no initial excess of 

 current when the heater switch is thrown on. Heating 

 coil enclosed in flattened steel sheath supported in calo- 

 rimeter. 



c. Potentiometer and Galvanometer. 



Wolff high resistance dial tj'pe potentiometer. 



Leeds and Northrup type HN moving coil galvanometer. 



d. Batteries. 



Twenty type B6H, 112.5 amp. hr. Edison Cells, used 

 solely for supplying energy to the calorimeter. 



e. Standard Cell. 



Weston Cadmium Cell, certified by the Bureau of Stand- 

 ards. 



f. Standard Resistance. 



1 ohm. N. B. S. Standard Resistance furnished and certi- 

 fied by Leeds and Northrup. 



g. Chronograph. 



Ordinary pen and drum type, regulated by laboratory 

 clock. Connected to heater switch in such manner as to 

 record automatically length of time heating current was on. 



3. Thermometric Apparatus. 



a. Standard Thermometer. 



Four lead compensated Callendar open coil type.^ 2 

 meters of 0.1 mm. diameter platinum wire wound on 



7 The vertical lacing was found to be an easy way to overcome the tendency 

 of the stiff manganin wire to twist the delicate mica supporting frame when 

 wound on in the ordinary manner. Holes were punched in the frame with a 

 needle at proper intervals and the wire threaded through these holes. 



8 "Measurement of High Temperatures," Burgess and Le ChateUer. John 

 Wiley and Sons. (1912), p. 202. 



