Blasting, Electric.
The ignition of blasting charges of powder or high explosives by the
electric spark, or by the ignition to incandescence (red or white heat)
of a thin wire immersed in or surrounded by powder. Special influence or
frictional electric machines or induction coils are used to produce
sparks, if that method of ignition is employed. For the incandescent
wire a hand magneto is very generally employed. (See Fuse, Electric.)
The cuts, Figs. 62 and 63, show one form of incandescent wire fuse. The
large wires are secured to the capsule, so that no strand can come upon
the small wire within the cavity.
The cut, Fig. 64, shows a frictional electric machine for igniting spark
fuses.
Bleaching, Electric.
Bleaching by agents produced or made available by the direct action of
electricity. Thus if a current under proper conditions is sent through a
solution of common salt (sodium chloride), the electrodes being close
together, the salt is decomposed, chlorine going to one pole and sodium
hydrate to the other. The two substances react upon each other and
combine, forming sodium hypochlorite, which bleaches the tissue immersed
in its solution.
Block System.
A system of signalling on railroads. The essence of the system consists
in having signal posts or stations all along the road at distances
depending on the traffic. The space between each two signal posts is
termed a block. From the signal posts the trains in day time are
signalled by wooden arms termed semaphores, and at night by lanterns.
The arms may be moved by hand or by automatic mechanism depending in
part on electricity for carrying out its functions. Thus in the
Westinghouse system the semaphores are moved by pneumatic cylinders and
pistons, whose air valves are opened and shut by the action of solenoid
magnets, q. v. The current of these magnets is short circuited by
passing trains, so as to let the valves close as the train passes the
signal post. The block system causes the semaphore to be set at "danger"
or "caution," as the train enters the next block. Then the following
train is not allowed to enter the block until the safety signal is
shown. The Westinghouse system provides for two semaphores on a post,
one indicating "danger" as long as the train is on the next block; the
other indicating "caution" as long as the train is on the next two
blocks. The rails form part of the circuit, their joints being bridged
by copper wire throughout the block, and being insulated where the
blocks meet.
Block Wire.
In the block system a wire connecting adjacent block-signal towers or
semaphore poles.
Blow-pipe.
A name sometimes given to an electric experiment illustrating the
repulsion of electrified air particles from a point held at high
relative potential. A metallic point, placed on the prime conductor of
an electric friction or influence machine, becomes highly electrified,
and the air becoming excited is repelled and acts upon the candle flame.
If the candle is placed on the conductor and a point held towards it the
repulsion is still away from the point.