850 
The decade resistance box Bé') brings the current of an accumu- 
lator Ar to the strength at which the thermo-current is compen- 
Wa--\2 Sz 
Th Pt Fe Ct ip bs 
Lp 
é 
Fig. 1. 
ated. Then the galvanometer Gr is without current. The circuits 
of accumulator and thermopile have the manganin wire AB, of about 
0.1 ohm, in common. Shunt St protects the galvanometer Gr. against 
too strong currents. The zero-position is obtained by bringing valve 
Kp before the lamp, and by interrupting the current with the con- 
tact key S/. 
The blackening measurements are made in the following way: 
We place the unblackened part of the photographic plate Pt in the 
light path of the lamp Lp and determine the resistance R, in resis- 
tance box Bk necessary to compensate the thermocurrent. Then we 
shift Pé, till the blackened part of the plate is in the light path, 
and compensate again, let us suppose with A, Ohm. The blackening 
R 
is then given by log = log B 
In the measurements the following sources of error should be 
taken into account: 
1. The spectral distribution of energy of the lamp (Lp) changes 
on protracted use. The same blackenings measured with an interval 
of some hours’ burning of the lamp never gave differences greater 
than the accuracy allowed by the method. 
1) Every resistance of from 0,1 to 100.000 ohms can be inserted on the 
decade resistance box. 
