Four wire and five wire systems have been more or less used, based on
identical considerations, and involving in each case the coupling of
three or of four dynamos respectively, or else employing a dynamo with
special armature connections to give the requisite three-fold or
four-fold division of total potential. In the five wire system the total
voltage is four times that of a single lamp, the lamps are arranged four
in series across the leads and the central wire is the only one that can
be considered a neutral wire. When lamps are burning entirely from three
side-leads they constitute a sort of three wire system by themselves,
and their central wire may for the time be a neutral wire.
In some of the three wire mains, especially in the larger sizes, the
neutral wire is made of much smaller section than that of a lateral
conductor, because in extensive districts it is practically impossible
that the current should be concentrated in the neutral wire.
Throw.
In a galvanometer the instantaneous deflection of the needle when the
contact or closing of the circuit is instantaneous, or when the
discharge is completed before the needle begins to move. The throw of
the needle is the datum sought when the ballistic galvanometer is used.
Synonym--Elongation.
Throw-back Indicator.
A drop annunciator, whose shutter or drop is electrically replaced.
Thrust-bearings.
Bearings to support the end-thrust or push of a shaft. In disc armatures
where the field-magnets attract the armatures in the direction of their
axis of rotation, thrust-bearings have to be provided. In ordinary
cylinder or drum armatures end-thrust is not applied, as a little end
motion to and fro is considered advantageous as causing more even wear
of the commutator surface.
Thunder.
The violent report which, as we hear it, succeeds the lightning flash in
stormy weather. It is really produced simultaneously with the lightning
and is supposed to arise from disturbance of the air by the discharge.
The rolling noise has been attributed to successive reflections between
clouds and earth, and to series of discharges reaching the ear from
different distances and through air of varying density. The subject is
obscure. By timing the interval from lightning flash to the report of
the thunder an approximate estimate of the distance of the seat of
discharge can be made. The first sound of the thunder should be timed.
An almost concurrence of thunder and lightning indicates immediate
proximity of the discharge.
[Transcriber's note: The speed of sound at sea level is about 5 seconds
per mile.]
Ticker.
A colloquial name for a stock or market report automatic printing
telegraph, which prints its quotations and messages on a long tape.