hich carries a short focus lens N. The light from M passes through
the magnets, and a magnified image of the quartz thread is thrown upon the
ebonite screen J. This screen is provided with a fine slit, and when the
galvanometer is at rest the shadow of the thread just covers the slit in
the screen and prevents any light from M reaching the photographic film.
Upon signals being received the shadow of the thread moves to one side for
a long or short period, uncovering the slit, and allowing light to pass
through. The lens R concentrates the collected light to a point upon the
revolving film. The connections for the complete receiver are given in Fig.
24.

The modified form of the Einthoven galvanometer, as arranged by Professor
Korn for use with his selenium machines for photo-telegraphy over ordinary
land lines, consists of two fine silver wires which are displaced in a
lateral direction between the pole pieces when traversed by a current; the
current passing through both wires in the same {46} direction. A small
shutter of aluminium foil is attached to the wires at the optical centre.
The silver wires used are 1/1000 inch in diameter, with a natural period of
about 1/120th of a second; the length of wires free to swing being usually
about 5 cm.

[Illustration: FIG. 24.]

The period of the wires depends to a great extent upon their length and
diameter, and also upon their tension. By using short fine wires the period
can be made much smaller, but a greater current is required to produce a
similar displacement. Where the current available, as in wireless
telegraphy, is very small, and a definite displacement of the wires is
required, it is at once apparent that with wires of a given diameter there
is a limit to their length and therefore to the period. Finer wires can be
used, but here again there is a practical limit to their fineness, although
galvanometers have been constructed with a single silvered quartz thread
1/12000th of an inch diameter, which, when placed in a powerful field, will
give a good displacement with a current as small as 10^{-8} ampere. {47}

With the apparatus arranged by the Poulsen Company, given in the diagram,
Fig. 17, for photographically recording wireless signals, the current
required to operate the galvanometer for signals transmitted at the rate of
1500 a minute is 1 x 10^{-6} ampere, while for signals up to 2500 a minute
a current about 5 x 10^{-6} ampere is necessary.

Another very se