The Standard Electrical Dictionary Part 82

Hermetically Sealed.

Closed absolutely tight. Gla.s.s vessels, such as the bulbs of incandescent lamps, are hermetically sealed often by melting the gla.s.s together over any opening into their interior.

Heterostatic Method.

A method of using the absolute or attracted disc electrometer. (See Electrometer Absolute.) The formula for its idiostatic use, q. v., involves the determination of d, the distance between the suspended and fixed discs. As this is difficult to determine the suspended disc and guard ring may be kept at one potential and the lower fixed disc is then connected successively with the two points whose potential difference is to be determined. Their difference is determined by the difference between d and d', the two distances between the discs. This difference is the distance through which the micrometer screw is moved. The heterostatic formula is:

V' - V = (d' - d)* squareRoot( 8*PI*F / S )

in which V and V' are potentials of the two points; d' and d the two distances between the discs necessary for equilibrium; S the area of the disc and F the force of attraction in dynes. (See Idiostatic Method.)

High Bars of Commutator.

Commutator bars, which in the natural wear of the commutator, project beyond the others. The surface then requires turning down, as it should be quite cylindrical.

High Frequency.

A term used as a noun or as an adjective to indicate in an alternating current, the production of a very great number of alternations per unit of time--usually expressed as alternations per second.

Hissing.

A term applied to a noise sometimes produced by a voltaic arc; probably due to the same cause as frying, q. v.

Hittorf's Solution.

A solution used as a resistance. It is a solution of cadmium iodide in amylic alcohol. Ten per cent. of the salt is used. It is contained in a tube with metallic cadmium electrodes. (See Resistance, Hittorf' s.)

Fig. 196. HITTORF'S RESISTANCE TUBE.

290 STANDARD ELECTRICAL DICTIONARY.

Holders.

(a) The adjustable clamps for holding the armature brushes of dynamos and motors.

(b) The clamps for holding the carbons of arc lamps.

(c) The clamps for holding safety fuses, q. v.

(d) Holders for Jablochkoff candles and other electric candles. (See Candle Holders.)

(e) A box or block of porcelain for holding safety fuses.

Hood.

A tin hood placed over an arc-lamp. Such hoods are often truncated cones in shape, with the small end upwards. They reflect a certain amount of light besides protecting the lamp to some extent from rain.

Horns.

The extensions of the pole pieces of a dynamo or motor. (See Following Horns-Leading Horns.)

Synonym--Pole Tips.

Horse Power.

A unit of rate of work or activity. There are two horse powers.

The British horse power is equal to 33,000 pounds raised one foot per minute, or 550 foot pounds per second, or 1.0138 metric horse power.

The metric horse power (French) is equal to 75 kilogram-meters, or 542 foot pounds per second, or .986356 British horse power.

H. P. is the abbreviation for horse power. (See Horse Power, Electric.)

Horse Power, Actual.

The rate of activity of a machine, as actually developed in condition for use. It is less than the indicated or total horse power, because diminished by the hurtful resistances of friction, and other sources of waste. It is the horse power that can be used in practise, and which in the case of a motor can be taken from the fly-wheel.

Horse Power, Electric.

The equivalent of a mechanical horse power in electric units, generally in volt-amperes or watts; 745.943 watts are equivalent to the activity of one British horse power; 735.75 are equivalent to one metric horse power. The number 746 is usually taken in practical calculations to give the equivalency.

[Transcriber's note: Contemporary values are: Mechanical (British) horsepower = 745.6999 Watts; Metric horsepower = 735.49875 Watts]

Horse Power, Indicated.

The horse power of an engine as indicated by its steam pressure, length of stroke, and piston area, and vacuum, without making any deduction for friction or hurtful resistances. The steam pressure is in accurate work deduced from indicator diagrams.

Horse Power, Hour.

A horse power exerted for one hour, or the equivalent thereof. As the horse power is a unit of activity, the horse power hour is a unit of work or of energy. It is equal to 1,980,000 foot pounds.

H. P.

Abbreviation for "horse power."

291 STANDARD ELECTRICAL DICTIONARY.

Hughes' Electro-magnet.

A horseshoe electro-magnet with polarized core. It is made by mounting two bobbins of insulated wire on the ends of a permanent horseshoe magnet. It was devised for use in Hughes' printing telegraph, where very quick action is required. The contact lasts only .053 second, 185 letters being transmitted per minute.

Fig. 197 HUGHES' ELECTRO-MAGNET.