Boll.
An absolute, or c. g. s., unit of momentum; a gram moving at the rate of
one centimeter per second; a gram-kine (see Kine); a unit proposed by
the British Association.
Bolometer.
An apparatus for detecting small amounts of radiant energy (radiant
heat, so called). A coil suspended by a fine wire or filament so as to
be free to rotate under the effect of force is made up of two parallel
and equal wires, insulated from each other, but connected so that
parallel currents sent through them go in opposite direction through
each. This coil is hung in a strong electro-magnetic field produced by a
large coil surrounding it. When a current passes through the suspended
coil no effect will follow, because the oppositely wound portions
counteract each other exactly. In the circuit with one half of the
suspended coil is an exceedingly thin strip of platinum wire. The other
half of the coil has no strips. Both halves unite after leaving the
coil. If now the strip of platinum is heated its conductivity is
affected and its half of the coil receives less current than the other
half. This disturbs the balance and the coil swings through a small arc.
This apparatus may be made very sensitive, so that an increase of
temperature of 1/1400º F., 9/70000°C. (1/14000º F.) will be perceptible.
Another construction takes the form of a Wheatstone Bridge, q. v., in
whose arms are introduced resistances consisting of bands of iron, .5
Millimeter wide (.02 inches), .004 millimeter (.00016 inch) thick, and
folded on themselves 14 times so as to make a rectangular grating, 17 x
12 millimeters (.68 x .48 inch). The least difference of heat applied to
the grating affects the galvanometer.
Synonym-Thermic Balance.
Boreal Pole.
The south pointing pole of the magnet. (See Austral Pole.)
Bot.
A colloquial expression for the English Board of Trade unit of
Electrical Supply. It is formed of the initials of the words "Board of
Trade." (See Unit, Board of Trade.)
Box Bridge.
A constriction of Wheatstone's Bridge in which the necessary resistance
coils are contained in a single box with plugs for throwing the coils in
and out of circuit, and connections to bring the coils into the
different arms of the system. The cut shows a box bridge. Connections
for the galvanometer, battery wires, and terminals of the unknown
resistance are provided, by which its resistances and the connections
are brought into the exact relations indicated in the conventional
diagram of Wheatstone's bridge. (See Wheatstone's Bridge.)
Referring to the cut, the battery wire, say from the zinc plate,
connects at A1, thereby reaching A, its true connecting point. To B1 one
end of the galvanometer circuit or lead is attached, thereby reaching B,
its true connecting point. To C are connected the other end from the
galvanometer and one end of the unknown resistance. The other end of the
unknown resistance, and the other end of the battery wire, in this case
from the carbon plate, connect to D. At G is an infinity plug, as it is
called. When out it breaks the circuit.
In use after the connections are made the key is depressed and the
galvanometer observed. The resistance is changed until no action of the
galvanometer is produced by closing the circuit when the ratio of the
resistances of the arms gives the proportion for calculating the unknown
resistances.
Synonym--Commercial Wheatstone Bridge, or commercial form of same.
Fig. 65. TOP OF BOX BRIDGE.