THE; CALORIMETER SYSTEM AND MEASUREMENT OF HEAT. 135 



The resistance of pure copper wire to the passage of an electric cur- 

 rent increases proportionally with the temperature. By means of a 

 Wheatstone bridge and a delicate galvanometer it is possible to meas- 

 ure with great accuracy the changes in resistance of a coil due to tem- 

 perature fluctuations. Where, as in this case, temperature differences 

 rather than absolute temperatures are involved, the problem becomes a 

 comparatively simple one. 



Inasmuch as the average temperature of the whole mass of air inside 

 the chamber is desired, it is necessary to distribute the coils in such 

 manner that the variations in resistance will represent as closely as 

 possible the actual temperature fluctuations of the air. For this pur- 

 pose the amount of wire the resistance of which is to be measured is 

 wound on five separate coils connected in series and suspended from 

 hooks at different points on the walls of the chamber. Whatever local 

 temperature fluctuations there may be in the different parts of the 

 chamber, each coil will rapidly acquire the temperature of the air im- 

 mediately surrounding it, and consequently the average variations of 

 the five coils taken as a whole will closely approximate the average 

 temperature fluctuations of the air inside the chamber. 



The coils consist of No. 32 pure copper wire, double-silk covered. 

 They are wound on wooden frames and are well protected by metal 

 guards. The total resistance of the coils is not far from 20 ohms. 

 Variations in resistance which indicate temperature changes of 0.01 

 are easily detected by the Wheatstone bridge and galvanometer used in 

 connection with these coils. 



These coils (T,, fig. 33), suspended as they are about 2.5 cm. from 

 the copper wall, acquire the temperature of the air rather than that of 

 the copper wall itself. In determining accurately the temperature 

 changes of the whole mass of material, it is frequently desirable to 

 know the temperature changes, not only of the air, but of the copper 

 wall. For this purpose four copper coils (T w , fig. 33) , having an aggre- 

 gate resistance of about 20 ohms, are wound on wooden frames and 

 slipped into copper pockets made by soldering a copper box to the copper 

 wall. These coils, therefore, are likely to assume the temperature of the 

 copper wall rather than that of the air. The temperature changes can 

 be detected by means of the switch, galvanometer, and bridge as closely 

 as suggested above for differences in temperature of the air. 



Variations in resistance of the copper coils are detected by means of 

 a Wheatstone bridge and a galvanometer. For many years we have 

 used a D' Arsonval galvanometer constructed by O. S. Blakeslee in the 

 mechanical laboratory of Wesleyan University. This instrument is 

 very sensitive and dead beat, allowing 10 readings each minute. The 



