( 5 mm. , .2 inches )
0.9985 (Ayrton)
0.99941 (Boltzman.)
The specific gravity of air under standard conditions 15.5° C (60° F.)
and 760 mm. barometric pressure (30 inches) is taken as unity as a
standard for gases.
[Transcriber's note: Argon accounts for 0.9340%. It was discovered in
1894, two years after this book.]
Air-Blast.
(a) In the Thomson-Houston dynamo an air-blast is used to blow away the
arc-producing spark liable to form between the brushes and commutator.
It is the invention of Prof. Elihu Thomson. The air is supplied by a
positive action rotary blower connected to the main shaft, and driven
thereby. The wearing of the commutator by destructive sparking is thus
prevented.
A drum H H is rotated, being mounted on the axis X of the dynamo. As it
rotates the three vanes are thrown out against the irregular shaped
periphery of the outer case T T. The arrow shows the direction of
rotation. The air is thus sent out by the apertures a a. O is the
oil-cup.
(b) The air-blast has also been used by Prof. Thomson in experiments
with high frequency currents of high potential. By directing a blast of
air against a spark discharge between ball terminals of an alternating
current, the nature of the current was changed and it became capable of
producing most extraordinary effects by induction.
14 STANDARD ELECTRICAL DICTIONARY.
Fig. 5. AIR BLOWER FOR THOMSON'S DYNAMO.
Air Condenser.
A static condenser whose dielectric is air. The capacity of an air
condenser in farads is equal to
A / ( 4.452E12 * t )
in which A is the area of one sheet or sum of the areas of one set of
connected sheets in square inches and t is the thickness of the layer of
air separating them.
A convenient construction given by Ayrton consists in a pile of glass
plates P separated by little bits of glass F of known thickness, three
for each piece. Tin-foil T is pasted on both sides of each piece of
glass and the two coatings are connected. The tin-foil on each second
plate is smaller in area than that on the others. The plates are
connected in two sets, each set comprising every second plate. For A in
the formula the area of the set of smaller sheets of tin-foil is taken.
By this construction it will be seen that the glass does not act as the
dielectric, but only as a plane surface for attachment of the tin-foil.
Posts E E keep all in position. One set of sheets connects with the
binding post A, the other with B.
The capacity of any condenser with a dielectric of specific inductive
capacity i is given by the formula:
( i *A^1 ) / ( 4.452E12 * t1 )
The air condenser is used for determining the value of i for different
dielectrics.
Fig. 6. AIR CONDENSER.
15 STANDARD ELECTRICAL DICTIONARY.
Air Gaps.
In a dynamo or motor the space intervening between the poles of the
field magnet and the armature. They should be of as small thickness, and
of as extended area as possible. Their effect is to increase the
magnetic reluctance of the circuit, thereby exacting the expenditure of
more energy upon the field. They also, by crowding back the potential
difference of the two limbs, increase the leakage of lines of force from
limb to limb of the magnet.
Air Line Wire.
In telegraphy the portion of the line wire which is strung on poles and
carried through the air.
Air Pump, Heated.
It has been proposed to heat portions of a mercurial air pump to secure
more perfect vacua, or to hasten the action. Heating expands the air and
thus produces the above effects.
16 STANDARD ELECTRICAL DICTIONARY.
Air Pump, Mercurial.
An air pump operated by mercury. The mercury acts virtually as the
piston, and the actuating force is the weight of the column of mercury,
which must exceed thirty inches in height. There are many types.
Mercurial air pumps are largely used for exhausting incandescent lamp
chambers. (See Geissler Air Pump,--Sprengel Air Pump.)
Air Pumps, Short Fall.
A mercurial air pump in which the fall of mercury or the height of the
active column is comparatively small. It is effected by using several
columns, one acting after the other. A height of ten inches for each
column suffices in some forms. Enough columns must be used in succession
to make up an aggregate height exceeding 30 inches.
Fig. 7. BURGLAR ALARM SWITCH OR CIRCUIT BREAKER.
Fig. 8. BURGLAR ALARM SWITCH OR CIRCUIT BREAKER.
Alarm, Burglar.
A system of circuits with alarm bell extending over a house or
apartments designed to give notice of the opening of a window or door.
As adjuncts to the system the treads of the stairs are sometimes
arranged to ring the bell, by completing a circuit when trod on. Door
mats are also arranged to close circuits in like manner.
17 STANDARD ELECTRICAL DICTIONARY.
For doors and windows switches are provided which are open as long as
the door or window is closed, but which, on being released by opening
the door or windows, automatically close the circuit. The circuit
includes an alarm bell and battery, and the latter begins to ring and
continues until stopped, either by the closing of the door or by a
switch being turned. The connections are sometimes so contrived that the
reclosing of the door or window will not stop the bell from ringing.
The cuts show various switches for attachment to doors and windows. It
will be seen that they normally keep the circuit closed, and that it is
only open when pressure, as from a closed door, is brought upon them. In
the case of a door a usual place for them is upon the jamb on the hinge
side, where they are set into the wood, with the striking pin
projecting, so that as the door is closed the pin is pressed in, thus
breaking the circuit.
Sometimes the connections are arranged so as to switch on the electric
lights if the house is entered. Special annunciators showing where the
house has been entered are a part of the system. A clock which turns the
alarm on and off at predetermined hours is also sometimes used.
The circuits may be carried to a central station or police station. One
form of burglar alarm device is the Yale lock switch. This is a contact
attached to a Yale lock which will be closed if the wrong key is used,
completing a circuit and ringing a bell.