ter may call for the subscriber by stating her wish
over the call circuit associated with the long-distance trunk. The
connection having been made by the switching operator, the long-distance
operator may withhold ringing the subscriber's bell until all is in
readiness for the conversation.

_High-Voltage Toll Trunks._ In some systems, the long-distance trunks
are further specialized by being enabled to furnish transmitter current
to subscribers at a higher voltage than is used in local conversations.
With a given construction of transmitters there is a critical maximum
current which can be carried by the granular carbon of the instrument
without excessive heating, consequent noises, and permanent damage. The
shortest lines and the longest lines of an exchange district being
served by a source of current common to all, the standard potential of
this source must be such as to give the longest lines current enough
without giving the shortest lines too much. The very longest local
lines, however, do not receive current enough from the standard
potential to give maximum efficiency when talking over long distances,
though they get enough for local conversations. By providing a battery
with a voltage twice that used for local conversations and connecting it
into the current supply element of the toll trunk through non-inductive
resistances, not too much current may be given to the shortest lines and
considerably more than normal current to the longest lines.

=Ticket Passing.= When only one operator is necessary in a town, her
duty being to switch both local and long-distance lines, she may write
her own tickets and execute them entire. In larger communities with
larger long-distance traffic, the duties need to be specialized. The
subscribers' wants as to long-distance connections are given by
themselves to recording long-distance operators, who write them on
tickets and pass these to operators who get the parties together. The
problem of ticket-passing becomes important and many mechanical carriers
have been tried, culminating in the system which utilizes vacuum tubes.
This is in some ways similar to vacuum or compressed-air tube systems
for carrying cash in retail stores. The ticket is carried, however,
without any enclosing case and the tubes are flat instead of round, _i.
e._, they are rectangular in section. By suitable means a vacuum is
maintained in a large common tube having a tap to a box-like valve at
each line ope