MEASUREMENT OF THE CONDUCTIVITY OF ELECTROLYTES 413 
to be used. Washburn is of the opinion that to avoid undue polariza- 
tion in the conductivity cell, a frequency of 1,000 cycles per second is 
necessary. Moreover Taylor and Acree have shown that as the fre- 
quency approaches infinity, variations in the resistance and capacity 
of the cell approach zero. If we adopt this high frequency, the only 
available type of detector is the telephone receiver. On the other 
hand, Hibbard and Chapman, after exhaustive experimentation with 
lower frequencies, assert that at 60 cycles per second, using a cell with 
platinized electrodes, the amount of polarization is practically negligible 
in all but a few exceptional types of solutions. If this is the case the 
plant physiologist may feel secure in using this frequency, which has 
several great advantages over higher frequencies as will now be 
explained. An additional security rests in the fact that polarization, 
if present, is easily detected in the "creeping" of the balance point, 
and can be immediately remedied by cutting down the amount of 
current and the period of time in which the circuit is closed. The 
main advantage in using a low frequency lies in the fact that another 
detector than the telephone may be used. Such a detector is the 
alternating current galvanometer, of which there are two general 
types. The advantages of such a substitution are many and are fully 
discussed by Hibbard and Chapman. All who have worked with the 
telephone as detector will understand the difficulties attending the 
constant strain of listening, and will appreciate the substitution of a 
method which enables sight to take the place of hearing. 
At this point the alterations in apparatus devised by the writer 
may properly be considered. If a frequency of 60 cycles per second is 
possible without a sacrifice in precision of comparative results, there 
should be some source of current more available and entailing less 
initial cost than the rotary converter. Such a source of current is 
present in practically every laboratory, and needs only to be reduced 
to the proper E.M.F. and potential. This is the ordinary iio-volt 
alternating-current lighting circuit. As supplied to the laboratory it 
is practically always a single-phase, 6o-cycle system, having in most 
cases a frequency variation of not more than one percent and a re- 
markably pure wave form. Taylor and Acree in their article have 
inserted oscillograms of the Madison (Wis.) city current, which are 
by no means exceptional, and can be duplicated elsewhere. For 
example when this type of current is used to supply a bridge network 
in which the bridge-wire has a resistance of 1.2 ohms, the current 
