iving instrument, and the speed of the motor is regulated until
this line lies close against a line drawn across the electrolytic paper.
Although this may seem an ideal method there are one or two considerations
to be taken into account. Unless the decomposition marks are made the
correct length and are properly spaced, however good the isochronising may
be, the result will be a blurred image. Any one who has worked with a
selenium cell, will know that it cannot change from its state of high
resistance to that of low resistance with infinite rapidity, and the
effects of this inertia, or "fatigue" as it has been called, are more
pronounced when working at a high speed. In working, the effects of this
inertia would be to increase the time of contact of the relay F (Fig. 30)
as the current from D would flow for a slightly longer period through R to
F than the period of {65} illumination allowed by K. This, of course, would
mean a lengthening of the marks on the paper; results would also differ
greatly with different selenium cells. There is a method of compensation by
which the inertia of a cell can almost entirely be overcome, but it would
add greatly to the complicacy of the receiving apparatus.

In using an electro-motor with any optical method of receiving there are
two methods available. The first is an arrangement similar to that used by
Professor Korn in his early experiments with his selenium machines. The
motor used for driving has several coils in the armature connected with
slip rings, from which an alternating current may be tapped off; the motor
acting partially as a generator, besides doing good work as a motor in
driving the machine. This alternating current is conducted to a frequency
meter, which consists of a powerful electro-magnet, over which are placed
magnetised steel springs, having different natural periods of vibration. By
means of a regulating resistance the motor is run until the spring which
has the same period as the desired armature speed vibrates freely. The
speed of the motors at both stations can thus be adjusted with a fair
amount of accuracy. Another method is to make use of a governor similar to
those employed in the Hughes printing telegraph system. A drawing of the
governor is given in Fig. 32. It consists of a

[Illustration] {67} metal frame which supports an upright steel bar S,
whose ends turn on pivots. This bar is rectangular in section. The
gear-wheel G is fastened near the botto