at the lines on the print cannot be wider apart, centre to
centre, than the lines of the screen used in preparing it. A good screen to
use is one having 50 lines to the inch, but one is perhaps more suitable
for experimental work a little coarser, say 35 lines to the inch. To use a
screen having 50 or more lines to the inch, the transmitting apparatus, as
will be evident later on, will require to be very nearly perfect.

[Illustration: FIG. 5.]

Before proceeding further it will perhaps be as well to make an experiment.
If we take one of the metal prints or, more simple, draw a sketch in
insulating ink upon a sheet of metal A, Fig. 5, and connect a battery B and
the galvanometer D as shown, we shall find on drawing the free end of the
wire across the metal plate that all the time the wire is in contact with
the lines of insulating material the needle of the galvanometer will remain
{17} at zero, but where it is in contact with the metal plate the needle is
deflected.

From this experiment it will be seen that we have in our metal line print,
which consists of alternate lines of insulating and conducting material, a
method by which an electric circuit can be very easily made and broken. It
is, of course, necessary to have some arrangement whereby the whole of the
surface of the metal print is utilised for this purpose to the best
advantage. One type of transmitting machine used for this purpose is
represented by the diagram, Fig. 6. The cylinder A is fastened to the steel
shaft B, which runs in the two bearings D and D', the bearing D' having an
internal thread corresponding to that on the shaft. The stylus in this
class of machine is a fixture, the cylinder being given a lateral as well
as a revolving movement. As it is impossible to use a rigid drive, a
flexible coupling F is employed between the shaft B and the motor.

[Illustration: FIG. 6.]

Another type of machine is shown in Fig. 7. The drum in this case is
stationary, the table T moving laterally by reason of the screwed shaft
{18} [Illustration] and half nut F. The table, shown separate in Fig. 8,
carries a stiff brass spring A, to which is attached a holder B made to
take a hardened steel point. The holder is provided with a set screw P for
securing the steel point Z. The spring and needle are insulated from the
rest of the machine, as shown in the drawing. In working, the metal print
is wrapped tightly round the cylinder of the machine, the glue image being