Air is a dielectric whose specific inductive capacity at atmosphere pressure is taken as 1. It is practically of exactly the same composition in all places and hence can be taken as a standard. When dry it has high resistance, between that of caoutchouc and dry paper.

Dampness increases its conductivity.

It is a mixture of oxygen and nitrogen, with a little carbonic acid gas and other impurities. Its essential composition is:

Oxygen: (by weight) 23.14 (by volume) 21 Nitrogen: 76.86 79

The specific inductive capacity varies for different pressures thus:

Approximate vacuum (.001 mm., .0004 inch) 0.94 (Ayrton) " " ( 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 gla.s.s plates P separated by little bits of gla.s.s F of known thickness, three for each piece. Tin-foil T is pasted on both sides of each piece of gla.s.s 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 gla.s.s 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 s.p.a.ce 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.

Fig. 9. BURGLAR ALARM SWITCH OR CIRCUIT BREAKER.

Alarm, Electric.