414 
NEWTON B. GREEN 
can be sufficiently reduced by the insertion in series of two i6 c.p. 
lamps. When the connections are made as in the diagram, the current 
passing through the bridge wire, if R and R' are open, is about .28 
amperes at a potential of .336 volt.^ In practice the resistances 
R and R' are in series with each other and the two connected in parallel 
with the bridge-wire. Then when the bridge is balanced, the current 
Fig. I. For explanation see text. 
divides itself between R -\- R' and the bridge-wire, so that the two 
divisions of the current are in inverse proportion to the resistances of 
these parallel branches. Since the resistance of the bridge-wire is in 
this case i .2 ohms and the resistance of the unknown {R') -j- its balance 
resistance (R) varies between 200 ohms for concentrated solutions and 
10^ ohms for conductivity water, it is easily seen that the amount of 
current passing through R' will be exceedingly small in all cases, 
running from .0016 ampere to 3.36 X lO"^ amp. If however heating 
occurs when a concentrated solution is being measured, it can easily 
be obviated by the introduction of a rheostat in series with the bridge- 
wire and lamps thus cutting the current down still further. 
There now remains the consideration of the detector. For^fre- 
^ The writer is indebted to Dr. Alan T. Waterman, of the Department of Physics 
of the University of Cincinnati, for assistance in calculating the electrical data. 
