THE TEMPERATURE OF LEAVES OF PINUS IN WINTER 43 
firm. The cell used to furnish the current for the potentiometer 
circuit (II, figure i) was an ordinary storage cell with an E.M.F. of 2 
volts. This cell proved entirely satisfactory, giving a uniform current 
throughout the experiments. As a check upon the cell, a Weston 
voltmeter was connected across the terminals of the potentiometer 
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circuit. By throwing a knife switch, the voltage applied to this 
circuit of constant resistance could be measured, and any variations 
or weakening of the cell detected. Variations could be corrected by 
varying the resistance of a small rheostat which was placed between 
the cell and the potentiometer. It was not found necessary to use it 
however. 
4. Thermo- Junctions. — The thermo- junctions were made of advance 
(a trade name for constantan, which is an alloy of copper and nickel) 
and copper wire obtained from the Driver, Harris Co., of Harrison, 
N. J. The advance wire was 0.07 mm. in diameter, the copper slightly 
