AN AUTOMATIC TRANSPIRATION SCALE OF LARGE CA¬ 
PACITY FOR USE WITH FREELY EXPOSED PLANTS 
By Lyman J. Briggs, Biophysicist in Charge , Biophysical Investigations , and H. L. 
ShanTz, Plant Physiologist Alkali and Drought Resistant Plant Investigations , 
Bureau of Plant Industry. 
INTRODUCTION 
An extended study of the transpiration rate of plants practically neces¬ 
sitates the use of an automatic balance of some type. The present paper 
contains a review of the various forms of transpiration balances hereto¬ 
fore employed, together with a description of a new automatic transpira¬ 
tion scale of large capacity, so designed that the plants may be freely 
exposed to the weather. Four of these scales have been in continuous 
use during the past four summers at Akron, Colo. 
Automatic balances may be divided into two classes: (i) The step- 
‘by-step type, in which small weights of equal value are added to the 
scale pan in succession or a counterpoise is advanced in equal steps; (2) 
the continuous record type, in which the plant is suspended from a spring 
or from a variable lever or is mounted directly on a float. 
RECORDING BALANCES OF THE STEP-BY-STEP TYPE 
Vesque (1878) 1 appears to have been the first to employ an automatic 
balance in measuring transpiration. He made use of the step-by-step 
principle, a measured quantity of mercury being delivered to a recep¬ 
tacle on the scale pan each time the beam tipped sufficiently to close an 
electric circuit. His apparatus is illustrated in figure 1, the device for 
measuring the mercury being shown at s and enlarged at B. This meas¬ 
uring device is in principle similar to a large stopcock, in which the plug 
is only partially bored through from each side so as to form two shallow 
cavities of equa^l volume. Either cavity in its upper position becomes 
filled with mercury from the reservoir t. When the circuit is closed, a 
spring motor is released, which turns the plug through one-half a revolu¬ 
tion, delivering the mercury in the cavity to the container a, and record¬ 
ing the time of the event by lowering the stylus p in contact with the 
circular plate v of the clock H. 
Anderson (1894) was the first to employ steel balls of uniform size as 
weights for a recording balance. The balls were held in a spiral brass 
tube, with a block at the lower end containing a pocket for one ball. 
When the balance beam tipped sufficiently to close an electric circuit, 
the block was moved sidewise and the ball in the pocket dropped into the 
1 Bibliographic citations in parentheses refer to "Literature cited/’ p. 131-132. 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
ftj 
Vol. V, No. 3 
Oct. 18, 1915 
G— 59 
