SPARK GAP:
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.