One of the very simplest light sources for exposing the film on the
receiving cylinder consists of a minute spark-gap located in contact
with the moving film.
The strength of the incoming current charges a small condenser until the
gap breaks down and the passing spark exposes the film (or perhaps
perforates it). If the current is strong the sparks pass the gap at a
high frequency, while if the current is weak the frequency is less. The
range may be from 500 to 5,000 per second perhaps, depending on the
current strength, and, of course, the film exposure correspondingly
varies, and the different degrees of density of the picture results.
This scheme requires about as small current as is likely to be
practical, perhaps, especially when the spark is in a suitable degree of
vacuum, and, of course, the incoming radio signals require
correspondingly small amplification.