IOWA ACADEMY OF SCIENCES 
221 
To obtain the recovery curve the operation is reversed after changing 
the galvanometer connecticns with a commutator. The curves of an old 
Samson cell are represented in Figure 2. 
In obtaining these curves the cell was placed on a closed circuit with 
an external resistance of 2.54 ohms including that of the primary coil. 
The maximum deflection obtained was 116 and represented a voltage of 
1.07. The line a b is the polarization curve, and the line e d is the curve 
of recovery. It will be noticed from the diagram that the recovery of 
the cell was slight in the time considered, indicating that the depolarizer 
was exhausted. 
The method also provides the data for the determination of M the 
coefficient of mutual induction of the coil from the equation MC over 
R"=kd. In this equation k stands for the constant of the galvanometer 
and d for the deflection. Consult Jackson’s Alternating Currents for this 
method. It is evident also, that, knowing the E. M. F. of any one cell, 
the method can be used to determine the voltage of any other with 
approximate accuracy. The method gives good results m determining 
voltages in cases where the internal resistance is low or negligeble com- 
pared to the external resistance of the primary circuit. 
The advantages of the use of the induction coil in such worK as has 
been suggested are as follows: 
First. The induction coil is inexpensive compared with the cost of a 
standard condenser. 
Second. The range of adjustment is far greater than that of the best 
subdivided condenser. The resistance in the primary circuit can be 
adjusted to secure any convenient deflection of the galvanometer. 
Third. The deflections of the galvanometer are practically unvaried. 
The differences in the successive readings are very small. With a con- 
denser the readings may vary considerably, particularly when the instru- 
ment is not of the best make. Some condensers seem to have a “soak- 
ing in” property which results in a variable charge, if the time of depres- 
sing and elevating the key during charge and discharge is not constant. 
