334 BABCOCK. 



dehydrated mica frame enclosed in glass tube. Leads 

 of 0.3 mm. platinum wire arc welded to coil terminals. 

 Leads to switch, ^ 20 B & S guage copper wire arc welded 

 to platinum leads and soldered to switch terminals. Re- 

 sistance at 0°C. 25.87 ohms. Used for calibrating observ- 

 ing thermometer by comparison, and for measuring temper- 

 ature of oil bath. 



b. Observing Thermometer. 



Four lead compensated Dickinson & Mueller flat coil type ^ 

 of special construction. 2 meters of 0.1 mm. platinum wire 

 wound on flat mica strip dehydrated by baking in electric 

 furnace at 900° C. Enclosed in flat steel sheath and built 

 into calorimeter. No drying agent was used. Top sealed 

 with china cement. Leads of # 20 B.&S. gauge copper wire, 

 arc welded to coil terminals and soldered to switch termi- 

 nals. Resistance at 0° C. 25.68 ohms. Used for measur- 

 ing temperature change in calorimeter. 



c. Wheatstone Bridge. Figure 5. 



This bridge is a modification of one designed by the Bureau 

 of Standards and built by the Leed's and Northrup Co.,^° 

 Range of Bureau of Standards instrument is 0° — 115° C. 

 Range of special bridge used in these measurements, 0° — 

 450° C. 



The instrument differs from the Bureau of Standards 

 design in three particulars: 



1. It is provided with 9, 11, 7, and 12 ohm extension 

 coils to increase the range of the instrument. 



2. A mercury cup contact link is inserted on the ther- 

 mometer side of the bridge to compensate the resist- 

 ance of the mercury cup link used to connect in the 

 extension coils. 



3. Thirteen instead of ten of the one ohm coils are used. 

 The extension coils cannot be adjusted to the same accuracy 

 as the 1 ohm coils when the bridge is built, but their value 

 may be determined in terms of the resistance of the slide 

 wire of the bridge, and within the limits of accuracy of the 

 bridge itself by the expedient of making a double setting 

 wuth a resistance thermometer at a constant temperature 

 and therefore at a constant resistance. With the ther- 



9 Dickinson and Mueller, Bull. Bur. of Standards, 3 (1907), p. 641. 

 10 Leeds and Northrup Bulletin, no. 806. 



