,
of course, uppermost. To fasten the print a little seccotine should be
applied to one edge, and the joint carefully smoothed down with the
fingers. [Illustration] If there is any tendency on the part of the print
to slip round on the drum, a couple of small spring clips placed over the
ends of the drum will act as a preventive. It is necessary to place the
print upon the drum in such a manner that the stylus draws away from the
edge of the lap and not towards it, and the metal prints should be of such
a size that when placed round the drum of the {19} machine a lap of about
3/16ths of an inch is allowed.

[Illustration: FIG. 9.]

The steel point Z (ordinary gramophone needles may be used and will be
found to answer the purpose admirably) is made to press lightly upon the
metal print, and while the pressure should be sufficient to make good
electrical contact, it should not be sufficient to cause the needle to
scratch the surface of the foil. The pressure is regulated by means of the
milled nut H. The electrical connections are given in Fig. 9. One wire from
the battery M is taken to the terminal T, and the other wires from M and F
lead to the relay R. The current flows from the battery M through the
spring Y, through the drum and metal print, the stylus Z, spring A, down to
the relay R, and from R back to the battery M. As the drum carrying the
single line half-tone print is revolved, the stylus, by reason of the
lateral movement given to the table or cylinder as the case may be, will
trace a spiral path over the entire surface of the print. As the stylus
traces over a conducting strip the circuit is completed, and the tongue of
the relay R is attracted, making contact with the stop S. {20} On passing
over a strip of insulation the circuit is broken and the tongue of the
relay R returns to its normal position.

As already stated, the conducting and insulating bands on the print vary in
width according to the density of the photograph from which it is prepared,
so that the length of time that the tongue of the relay R is held against
the stop S, is in proportion to the width of the conducting strip which is
passing under the stylus at any instant. The function of the transmitter is
therefore to send to the relay R an intermittent current of varying
duration.

The two photographs Figs. 10 and 10_a_ are of a machine designed and used
by the writer in his experiments. In this machine the drum is 3.5 inches
long and 1.