Clockwork and electro-motors are the source of driving power that are most suitable for photo-telegraphic work, and each has its superior claims depending on the type of machine that is being used. For general experimental work, however, an electro-motor is perhaps the most convenient, as the speed can be regulated within very wide limits. For a constant and accurate drive a falling weight has no equal, but the apparatus required is very cumbersome and the work of winding both tedious and heavy. This method of driving was at one time universally employed with the Hughes printing telegraph, but it has now been discarded in favour of electro-motors, which are more compact, besides being cheaper to instal in the first instance.
Synchronising and isochronising the two machines are the most difficult problems that require solving in connection with wireless photography, and as previously mentioned, the { 64} synchronising of the two stations must be very nearly perfect in order to obtain intelligible results. The limit of error in synchronising must be about 1 in 500 in order to obtain results suitable for publication.
The electrolytic system is perhaps the easiest to isochronise, as the received picture is visible. On the metal print used for transmitting, and at the commencing edge a datum line is drawn across in insulating ink. The reproduction of this line is carefully observed by the operator in charge of the receiving 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.