314 Journal of Agricultural Research voi. v, no. s 



integrate different temperatures if there are diflferences, and as the air 

 is very thoroughly stirred by the electric fan previously mentioned, it 

 is probable that the resistance thermometer shows the average tempera- 

 ture of the air of the chamber. In the interior view in Plate XXXV, 

 figure I, two of the frames are plainly shown with a wide-mesh wire 

 screen before the resistance wire to protect it against contact with any 

 object that would cause a short circuit between two parts of the wire, 

 as well as against injury. 



The six coils are connected in series by well-insulated No. 16 copper 

 wire, and similar wire leads from the terminals of the series, through a 

 rubber stopper in a small opening in one wall of the chamber, to a special 

 switch on the observer's table, by which they may be connected in one 

 arm of the Wheatstone bridge. The purpose of the switch is to provide 

 means for using with these coils the same bridge that is used with other 

 coils for measuring the temperature of the walls of the chamber and that 

 of the body of the subject, as explained later in this paper. This switch 

 must be designed to avoid the error that would result from introducing 

 appreciable resistance of the switch contacts into the bridge circuits. 

 The connections between the bridge and the resistance coils include a 

 compensating lead to eliminate from the measurement of the resistance 

 of the coils the effect of both the resistance of the leads and any change 

 in their resistance due to change of temperature. The contact that is 

 moved along the slide wire of the bridge, to restore balance when the 

 resistance of the thermometer coils has changed, is in series with the 

 battery, so that contact resistance introduces no error in the measure- 

 ment. 



The six coils have a total resistance of about 20 ohms at 20° C. Since 

 the resistance of nickel wire varies approximately 0.4 per cent per degree 

 at the usual temperatures of the experiments, their total change in resist- 

 ance would be close to 0.08 ohm for a change of 1° in the temperature of 

 the air of the chamber. The resistance of the slide wire of the Wheatstone 

 bridge will balance the bridge circuit for the change of resistance in the 

 coils that would result from a change of 5° in the temperature. By means 

 of several coils of manganin wire, which may be connected in series with 

 the slide wire, the total range of the bridge may be extended, but under 

 usual experimental conditions the temperature of the air is allowed to 

 change as little as possible. Whether the change is large or small, it 

 must be measured accurately. A change of resistance in the thermom- 

 eter coils resulting from a change of 0.01° in the temperature of the air 

 will upset the balance of the bridge sufficiently to cause a deflection of 

 the sensitive reflecting D'Arsonval galvanometer that indicates when the 

 bridge is balanced. The balancing point of contact may be moved along 

 the wire a distance sufficiently small to restore the balance, and the scale 

 of the slide wire will indicate the distance. 



