MEASUREMENT OF THE CONDUCTIVITY OF ELECTROLYTES 415 
quencies around 60 cycles the vibration galvanometer, as made by 
Leeds and Northrup for seventy-five dollars, is the most sensitive. 
It is easily tuned to the exact frequency of the current supply and once 
tuned needs attention only on rare occasions. Its sensitivity is such 
as to make profitable the use of the most accurate bridge with ex- 
tensions on the bridge-wire. An added advantage is that the moving 
coil returns quickly to its neutral position when the circuit is broken, 
so that the band of light from the mirror follows closely in its width 
the position of the slider on the wire. This enables a speedy deter- 
mination of the balance point and cuts down the chance of polarization. 
The total cost of such an apparatus using the best bridge, resis- 
tances, conductivity cell and condensers will be much less than either 
the Washburn or the Hibbard and Chapman outfits. This is made 
possible by the substitution of the city current for an expensive piece 
of apparatus, which is itself often a source of annoyance because of 
noise. Moreover the vibration galvanometer is less expensive than 
the electro-dynamometer type, and there is no sacrifice in precision. 
The writer believes that these advantages will appear to be of distinct 
importance to plant physiologists and to others interested in conduc- 
tivity measurements. 
Summary 
Since Messrs. Hibbard and Chapman have shown that polarization 
is in nearly all cases a negligible factor using a current of 60-cycle 
frequency, the ordinary single-phase, iio-volt, a.-c. lighting circuit 
can be used as a source of current in making measurements of the con- 
ductivity of electrolytes. 
With such a frequency the most sensitive and convenient detector 
is the vibration galvanometer. 
The use of this method in preference to those previously known 
enables the investigator, who desires only precise comparative results, 
to make a considerable saving in first cost of apparatus without any 
attendant sacrifice in accuracy. 
BIBLIOGRAPHY 
1. Kohlrausch, F. and Holborn, L. Das Leitvermogen der Elektrolyte. Leipzig, 
1898. 
2. Washburn, E. W. and Bell, J. E. An Improved Apparatus for Measuring the 
Conductivity of Electrolytes. Journ. Amer. Chem. Soc. 35: 174. 1913. 
.Hibbard, R. P. and Chapman, C. W. A Simplified Apparatus for Measuring 
the Conductivity of Electrolytes. Mich. Agr. Coll. Techn. Bull. 23. 1913. 
