140 
FIFTEENTH REPORT. 
maximum allowable current to be used is one-fifth of ampere for ten 
second periods. An ordinary resistance box can be introduced between 
the transformer and the fixed coil of the galvanometer, so that a total 
resistance of 60 ohms can be had, thus reducing the exciting current 
from the 12 volt transformer to one-fifth of an ampere. The resistance 
°f the swinging coil is 10 ohms and the maximum allowable current to 
be used is 1 10 of an ampere for ten second periods. By introducing 
a transformer of variable voltage from 0 to 9 volts, stepping up by 
increments of 3 /o volts, a maximum sensibility can be obtained providing 
the proper conductivity cells or cups are used. The switch on this 
transformer can be used to make and break the current for the time 
allowed. 
The bridge employed in these experiments is also a product of the 
Leeds and Northrop Company. It is the roller type supplied with a 
rheostat coil in the base. For very accurate work requiring at least 
and precision of .05 per cent, it is necessary to use the “extended” 
wire attachmenf. The smallest scale division on the bridge is 3 mm. 
wide, and in all but the dilute solution, the movement of the scale, in 
some cases scarcely the thickness of a line, caused a deflection in the 
galvanometer. For convenience, each scale division is divided into 
tenths, such divisions being easv to estimate with the eve. All measure- 
merits tvere made as near the middle of the bridge as possible and for 
this reason, a variable resistance was put in series with the known 
resistance so that a maximum sensibility could be reached. The Otto 
Wolf high grade standard manganin resistances were used in the pre- 
liminary experiments. The purer resistances as perfected by Dr. 
Curtiss (“Resistant Coils for Alternating Current Work, Bui. No. 3 
Vol. 8 Reprint 177, Bureau of Standards) to whose work we have 
already referred will be used in the final test. 
For the purpose of showing the degree of precision that has been 
attained, I mention below a few preliminary experiments. The cells 
used were the ordinary Ivohlrausch type and will be referred to as 
cell No. 1 and No. 2. With the aid of a cathetometer, the electrodes were 
measured. These were platinized according to the usual method, the 
cell constants determined and from this data the specific conductance 
of the various solutions calculated. During the measurements, the 
TABLES. 
SERIES I CELL NUMBER 1. 
Dilution of K Cl. 
Resistance in ohms. 
Dead point. 
ri 
11.27 
10 
• 
11 
102.83 
100 
n 
934 . 23 
+ 
1000 
n 
6547.34 
.5 scale division 
10000 
Conductivity 
24,872.00 
9 scale divisions 
water 
