714 Journal of Agricultural Research vol. vi. No. 18 



a small electric motor, is governed by the deflection of the pointer of a very 

 sensitive galvanometer, which is also incorporated in the mechanism, 

 with its terminals connected with the Wheatstone bridge. When the bridge 

 is in balance, the pointer remains at the zero position, and the slide wire is 

 not moved ; but any variation in the temperature of the water in either 

 thermometer results in a change of resistance in the thermometer coils that 

 upsets the balance of the bridge, the pointer swings to one side of the zero 

 position or the other, according to the relation between the resistances of 

 the opposite branches of the bridge, and the disk is turned in that direction 

 in which the slide wire should be moved to restore the balance of the 

 bridge. The amount of change in the position of the contact point on 

 the slide wire is proportional to the magnitude of the swing of the pointer, 

 which depends on the temperature difference in the thermometer coils. 

 A difference of 0.005 °, or even less, will upset the balance of the bridge 

 sufficiently to cause a swing of the galvanometer pointer that will result 

 in a movement of the disk. However large the temperature difference 

 might be at any given instant, because of certain mechanical details 

 connected with the mechanism for rotating the disk, stops are provided 

 to limit the swing of the pointer either side of zero to that which would 

 result from an inequality of resistance due to a difference of nearly 0.2 ° 

 in the thermometer coils; but the cam shaft rotates every 5 seconds, 

 and the disk may be moved that often if necessary; hence, the mechan- 

 ism will keep the bridge balanced for inequalities resulting from any 

 change of temperature difference in the ingoing and outgoing water 

 up to 2 per minute. 



The shaft on which the disk rotates also causes a pen to draw a line 

 on ruled paper to show the direction and the distance that the slide wire 

 had to be moved to balance the bridge. In the width of paper corre- 

 sponding to the length of the slide wire that is equal to a difference of 2 

 there are 100 lines. The distance from line to line, which represents a 

 temperature of 0.02 , is 2.5 mm. Hence, the temperature difference 

 indicated by the position of the pen at any instant may be easily read 

 to o.oi . 



The current in the bridge circuit is not sufficient to cause an increase 

 in the temperature of the thermometers that will produce a movement 

 of the pen even when the water is flowing through the thermometer at a 

 rate much lower than the lowest that would be used with the apparatus. 



A differential thermoelement is installed in the resistance thermom- 

 eters so that the temperature difference of the water in the bulbs may be 

 determined by means of a potentiometer as a check upon the measure- 

 ment by the recorder. The Wheatstone bridge is provided with duplicate 

 parts, which, by substitution, serve as means of checking the accuracy 

 and constancy of the resistances of the bridge. 



