326 Journal of Agricultural Research voi. v. no. 8 



methods before the resistance coils became short-circuited were always 

 in very satisfactory agreement, but this was hardly a sufficient test of 

 the accuracy of the resistance method, because the measurement of 

 temperature difference by the electric-resistance thermometers is much 

 superior to that by the mercury thermometers in sensitivity and pre- 

 cision. With the third type of resistance bulb a more satisfactory 

 method of checking was provided. A differential thermoelement, with 

 several junctions of copper and constantan wire in each end inclosed in 

 thin glass tubing, was mounted with one end in the water just leaving the 

 ingoing thermometer and the other end in the water just entering the 

 outgoing thermometer. The terminals of the thermoelement were 

 connected with binding posts on the observer's table, from which con- 

 nection could be made with a potentiometer, by means of which tem- 

 perature differences could easily be measured to an accuracy of o.oi 

 degree. Measurement of the increase in temperature of the water 

 flowing in the heat absorber by means of this apparatus afforded a real 

 check on the measurement with the resistance thermometer. 



MEASUREMENT BY TEMPERATURE DIFFERENCE RECORDER 



As a matter of fact, this method of measurement could be employed 

 instead of the resistance-thermometer method when the readings are to 

 be made and recorded by the observer. Either method was more con- 

 venient and decidedly more sensitive than the mercury thermometers, 

 and by use of it the temperature difference was actually measured to 

 0.01 degree, whereas in reading the mercury thermometers the tem- 

 perature was only estimated to 0.0 1 degree. The particular advantage 

 in the resistance thermometers was in the opportunity to use with them 

 a device which gives automatically a practically continuous record of the 

 difference between the temperature of the water entering and that of the 

 water leaving the heat absorber. A device of this character which has 

 been employed for five years in the investigations with the present res- 

 piration calorimeter has proved very satisfactory indeed and relieves the 

 observer of a considerable amount of drudgery, while it entirely elimi- 

 nates the possibility of error due to personal inaccuracy in recording 

 data regarding the temperature differences. 



Like the resistance thermometers described above, the two coils used in 

 this device have the same resistance, approximately 25.5 ohms, at the 

 same temperature, and the same change in resistance with the same change 

 of temperature, but the bulbs differ somewhat in mechanical construction 

 from the earlier type. The platinum resistance wire is not in a thin, flat 

 coil in a flat sheath, but is in a helical coil in a narrow annular space be- 

 tween two metal tubes with thin walls. The wire is wound upon the 

 inner tube, and the outer tube fits close against it, an electrical insulation 

 of thin sheet mica separating the wire from each tube. The space between 



