cells of the same size for any resistance between 10 and
1,000,000 ohms, and also, a cell may remain in good working condition for
several months, while another will become useless in as many weeks.

The ability of a cell to respond to very rapid changes in the illumination
to which it is exposed is determined largely upon its inertia, it being
taken as a general rule {111} that the higher the resistance of a cell the
less the inertia, and _vice versa_, and also, that the higher the
resistance the greater the ratio of sensitiveness. Inertia plays an
important part in the working of a cell, slightly opposing the drop in
resistance when illuminated, and opposing to a [Illustration] much greater
degree the return to normal for no-illumination. The effects of inertia or
"lag," as it is termed, can readily be seen by reference to Fig. 55. It
will be noticed that the current value rapidly increases when the cell is
first illuminated, but if after a short time _t_ the light is cut off, the
current value, instead of returning at once to normal for no-illumination,
only partially rises owing to the interference of the inertia, and some
time elapses before the cell returns to its normal condition; the time
varying from a few seconds to several minutes, depending upon the
characteristics of the cell and the amount of light to which it is exposed.
An actual curve is given in Fig. 55a. The inertia or "lag" of a cell
produces upon an intermittent current an effect similar to that produced by
the capacity [Illustration] of a line, as was noted in Chapter I.,
preventing the incoming signals from being recorded separately, and
distinctly. To obtain the best results in photo-telegraphy, the resistance
of a cell should only be decreased to an extent sufficient to pass the
current required to operate the recording apparatus, and the illumination
should be regulated so that this condition of the cell takes place.

The comparative slowness of selenium in responding to {112} any great
changes in the illumination offers a serious difficulty to its use in
photo-telegraphy, but various methods have been devised whereby the effects
of inertia can be counteracted. In the system of De' Bernochi (see Chapter
I.) the changes in the illumination are neither very rapid nor very great,
and the inertia effects would therefore be very slight; but in any
photo-telegraphic system in which a metal line print is used for
transmitting, where the source of illum