ination is constant and the
resistance of the cell is required to drop to a definite value and return
to normal instantly, many times in succession, the inertia effects are very
pronounced. The most successful method of counteracting the inertia is that
adopted by Professor Korn of always keeping the cell sufficiently
illuminated to overcome it, so that any additional light acts very rapidly.
Another method worked out and patented by Professor Korn, and known as the
"compensating cell" method, gives a practically dead beat action, the
resistance returning to its normal condition as soon as the illumination
ceases. The arrangement is given in the diagram Fig. 56.

[Illustration: FIG. 55a.]

Light from the transmitting or receiving apparatus, as the case may be,
falls upon the selenium cell S^1, which is {113} placed on one arm of a
Wheatstone bridge, a second cell S^2 being placed on the opposite arm. The
selenium cell S^1 should have great sensitiveness and small inertia, the
compensating cell S^2 having proportionally small sensitiveness and large
inertia. Two batteries B, B', of about 100 volts, are connected as shown, B
being provided with a compensating variable resistance W; W' is also a
regulating resistance. When no light is falling upon the cell S^1, light
from L is prevented from reaching the second cell S^2 by a small shutter
which is fastened to the strings of the Einthoven galvanometer (described
in Chapter III.), and the piece of apparatus C--relay or galvanometer as
the case may be--remains in a normal condition. When, however, light falls
upon the cell S^1, the balance of the bridge is upset, and light from L
falls a fraction of a second later upon the second cell S^2, and the
current flowing through C completes the circuit. Needless to say it is
necessary that the two cells be well matched, as it is very easy to have
over-compensation, in which case the current is brought below zero.

[Illustration: FIG. 56.]

It is also stated that by enclosing the cells in exhausted glass tubes,
their inertia can be greatly reduced and their life considerably prolonged.
The sensitiveness of a cell is the ratio between its resistance in the dark
and its resistance when illuminated. The majority of cells have a ratio
between 2:1 and 3:1, but Professor Korn has shown mathematically that by
conforming to certain conditions regarding the construction the ratio of
sensitiveness may be between 4:1 and 5:1. Thus a cell of R