ieces of
delicate apparatus all requiring careful adjustment add greatly to its
reliability.

[Illustration: FIG. 17.]

In any spark system with a properly designed aerial a coil taking ten
amperes is capable of transmitting signals over a distance of thirty to
fifty miles, but where the number of interruptions of the break required
per second is very high, as in radio-photography, it must be remembered
that a much higher voltage is needed to drive the requisite amount of
current through the primary winding of the coil than would be the case if
the interruptions were slower. It is possible to use platinum {34} contacts
for the relays, for currents up to ten amperes, but for heavier currents
than this some arrangement where contact is made with mercury will be found
to be more economical and reliable.

In the transmitter already described and given in Fig. 11, the best results
would be obtained by finding the speed at which the relay R' works best,
and regulating the number of contacts made by the stylus accordingly.

The method employed by De' Bernochi (see Chapter I.) of varying the
intensity of a beam of light by passing it through a photographic film,
which in turn alters the resistance of a selenium cell, has been very
successfully employed in at least one system of photo-telegraphy. Its
application has also been suggested for wireless transmission, and although
with any system using continuous waves this would not be very difficult, it
could hardly be adapted to work with the ordinary spark system. The
apparatus for receiving from this type of transmitter would, on the other
hand, necessarily be more elaborate than the methods that are described in
the next chapter, and as far as the writer's experience goes, experiments
along these lines would not prove very profitable, as simplicity is the
keynote of success in any radio-photographic system.

It has been suggested that in order to decrease the time of transmission a
cylinder capable of {35} taking a print 7 inches by 5 inches be employed,
the print being prepared from rather a coarse line screen--say 35 to the
inch--and a traverse of about 1/50 inch given to the stylus, thus reducing
the time of transmission to about twelve minutes. It is questionable,
however, whether the increase in speed would compensate for the loss of
detail, as only very bold subjects could be transmitted. As already pointed
out, wireless transmission would only be employed for fairly l