412 
NEWTON B. GREEN 
necessary in connection with the high frequency generator to maintain 
the correct resonance in the bridge circuit by means of a double con- 
denser. Consequently a complete outfit for making conductivity 
measurements, using the generator and excluding bridge, conductivity 
cell and resistance coils, which are of course necessary in any form of 
apparatus, would cost over two hundred and fifty dollars. If the oscil- 
lator is substituted as a source of current the price is increased to about 
three hundred dollars. In either case a condenser must be used to 
balance out the capacity in the conductivity cell. 
Hibbard and Chapman have met the problems as to source of 
current and detector in a different manner. They use a 6o-cycle 
rotary converter and an alternating current galvanometer of the 
electro-dynamometer type. The latter costs about one hundred 
dollars. In addition, a rheostat is necessary to regulate the primary 
current. Consequently this apparatus exclusive of bridge, cell and 
resistances must cost as much or more than that employed by Wash- 
burn, without being applicable to as wide a variety of conditions. 
The physical chemist will occasionally need a current of higher fre- 
quency than 60 cycles, and such variation is impossible with the 
ordinary rotary converter. Obviously the main objection to both of 
these methods from the standpoint of the plant physiologist is the one 
of expense involved. This is especially significant when one considers 
that the apparatus employed serves one purpose only in the plant 
physiology laboratory, namely, that of measuring changes in permea- 
bility, or the electrolytic content of plant tissues and juices. Con- 
sequently the writer considers it likely that an apparatus embodying 
the latest methods of procedure, which fulfills all the requirements of 
precision, will be welcomed by workers along this line. 
Since the investigators, whose results are cited above, have con- 
ducted the most exhaustive researches on the subject of conductivity 
measurements, it is certainly desirable to follow any procedure which 
they all recommend. We may assume th^n the necessity for a constant 
temperature bath, in which to immerse the conductivity cell, and a 
condenser to balance out the capacity in the cell. It is also certain 
that the Curtis coils are the most reliable of all available resistances, 
because they are so wound as to reduce inductance and capacity to a 
minimum. By standardizing the apparatus to this extent we are 
sure that the results obtained will have at least precise comparative 
values. We now come to the question of the source and type of current 
