220 
IOWA ACADEMY OF SCIENCES 
MINUTES. 
Fig. 2. Curve of Sampson Cell. 
switch. The shunt circuit contains a resistance box R, and a key K*. 
The operation consists, first, in closing the key K a moment and reading 
the deflection of the galvanometer. Then a second reading is obtained by 
closing the keys K and in quick succession and in the order named. 
The demonstration of the working formula depends on the principle 
that the deflection of the galvanometer is directly proportional to th^^ 
maximum current in the primary. This is evident since the quantity of 
electricity which passes through the galvanometer is equal to MC over 
R- in which M stands for the co-efficient of mutual induction, C for the 
current in the primary, R- for the resistance in the secondary. To sim- 
plify the demonstration let R\ the resistance in the shunt circuit, be 
an aliquot part of R, the resistance in the primary. Applying Ohm’s 
law and the principle of the divided circuit we get the following propor- 
tion: 
1.1 
d : ‘ (n+1) r+R 
R(d-di) 
S )lving, r=(n-fl)di-d 
In the above formula r stands for the internal resistance of the cell, 
R for the external resistance of the primary circuit, d and d^ for the two 
different deflections of the galvanometer and n denotes what aliquot part 
the shunt resistance is of the primary resistance. 
The method of the induction coil gives excellent results when used to 
obtain the polarization and recovery curves of a voltaic cell. In such 
a determination the primary circuit remains closed and the key is ele- 
vated at regular intervals to obtain the readings of the galvanometer. 
