122 



Journal of Agricultural Research 



Vol. V, No. 3 



its buoyancy. Temperature effects can, however, be practically elimi- 

 nated by surrounding the hydrometer tank with a water-jacket, through 

 which water is constantly circulating. The sensibility of the apparatus 

 is determined by the cross section of the stem of the hydrometer. 



A NEW AUTOMATIC TRANSPIRATION SCALE OF LARGE CAPACITY 



The requirements of the transpiration studies at Akron necessitated an 

 automatic weighing apparatus having a carrying capacity of 150 kgm., 



capable of operating 

 positively in the wind, 

 and so designed that the 

 plants could be freely 

 and continuously ex- 

 posed to the weather 

 (PI. IX). A platform 

 scale with agate bear- 

 ings having a carrying 

 capacity of 200 kgm. 

 and a sensibility of 5 gm. 

 was chosen for equip- 

 ment as an automatic 

 balance of the step-by- 

 step type (PL X). The 

 scale was fitted with a 

 short column so as to bring all the mechanism below the level of the top 

 of the pot and was provided with the following auxiliary equipment : 



a. Ball-dropping device. 



b. Ball receiver on beam. 



c. Beam contact and mercury cups. 



d. Oil dash pot on beam. 



e. Spring motor for raising beam. 



/. Adjustable counterpoise for raising the center of gravity of balanced system. 



g. Recorder for registering time at which each ball is dropped. 



h. Batteries and relay. 



i. Case for protecting mechanism from the weather. 



The beam of the scale with a part of the auxiliary equipment is shown 

 in fig. II. The operation of the mechanism is briefly as follows: As the 

 plant decreases in weight, the beam falls until an electric contact is 

 made at U. This closes a relay circuit, with the following results: 



1. The ball-dropping device A deposits a ball in the receiver L. The weight 



of this ball tends to raise the beam. 



2. The spring motor, by means of a cam K, raises the beam promptly and 



positively to its upper position. 



3. The time of the event is indicated on the drum of the recorder. 



-The transpiration balance of Richard Freres with its record- 

 ing apparatus. 



