nother, and are not always uniform as to material, it has not
been possible to secure as good telephone circuits from telegraph wires
as telegraph circuits from telephone wires.

Practical results are secured by adaptation of the original principle of
different frequencies. A study of Fig. 468 shows that over such a
composite circuit the usual method of ringing from station to station
over the telephone circuit by an alternating current of a frequency of
about sixteen per second is practically impossible. This is because of
the heavy short-circuit provided by the two 30-ohm choke coils at each
of the stations, the heavy shunt of the large condensers, and the
grounding through the 50-ohm choke coils. If high-frequency speech
currents can pass over these circuits with a very small loss, other
high-frequency circuits should find a good path. There are many easy
ways of making such currents, but formerly none very simple for
receiving them. Fig. 471 shows one simple observer of such
high-frequency currents, it being merely an adaptation of the familiar
polarized ringer used in every subscriber's telephone. In either
position of the armature it makes contact with one or the other of two
studs connected to the battery, so that in all times of rest the relay
_A_ is energized. When a high-frequency current passes through this
polarized relay, however, there is enough time in which the armature is
out of contact with either stud to reduce the total energy through the
relay _A_ and allow its armature to fall away, ringing a vibrating bell
or giving some other signal.

[Illustration: Fig. 471. Ringing Device for Composite Circuits]

Fig. 472 shows a form of apparatus for producing the high-frequency
current necessary for signaling. It is evident that if a magneto
generator, such as is used in ordinary magneto telephones, could be made
to drive its armature fast enough, it also might furnish the
high-frequency current necessary for signaling through condensers and
past heavy impedances.

[Illustration: Fig. 472. Ringing Current Device]

Applying these principles of high-frequency signals sent and received to
a single-wire telegraph circuit, the arrangement shown in Fig. 473
results, this being a type of railway composite circuit. The principal
points of interest herein are the insertion of impedances in series with
the telegraph lines, the shunting of the telegraph relays by small
condensers, the further shunting of the whole