374 STANDARD ELECTRICAL DICTIONARY.

In the original meter bridge the wire was one meter long, whence its name, and was stretched straight. In more recent examples the wire varies in length and in one form is bent into a circle or spiral, so as to make the instrument more compact.

The contact is not a sliding one, but is adjusted by trial. The contact piece is slid along, but not touching the wire, and from time to time is pressed down against the wire. This prevents wear of the wire. The wire may be made of platinum or of platinum-iridium alloy. The latter is very hard and not easily worn out.

Sometimes, as shown in the cut, three parallel wires are stretched along the baseboard of the instrument, and arranged so that a single wire, two wires or three wires in series can be used for the proportional sides of the bridge, thus making it a two-meter or three-meter bridge as desired.

On the other hand some are made of restricted length, as a half or quarter meter only.

Fig. 240. METER BRIDGE.

In the cut J K is the wire, traversed by the contact key. By moving the contact C back and forth in the slot it can be brought over any of the three divisions of the wire. H is the handle for depressing the key. S is a flat spring, carrying the contact piece and holding it up from the wires, except when pressed downwards. As shown in the cut, it is in use for calibrating a voltmeter V, by Poggendorff's method, G being the galvanometer and r1 and r2 being resistances.

Synonyms--Slide Bridge--Slide Balance.

Meter Candle.

A unit of illuminating power; the light given by one standard candle at a distance of one meter. The ordinary units of illuminating power are altogether relative; this one is definite.

375 STANDARD ELECTRICAL DICTIONARY.

Meter, Chemical Electric.

A current meter in which the current is determined by the amount of chemical decomposition which it can effect. In the Edison meter the solution is one of zinc sulphate. Two electrodes of zinc are immersed in it, and a fractional part of the current is pa.s.sed through it. The gain in weight of one electrode and the loss in the other are proportional to the current. Both electrodes are weighed periodically, one acting as check upon the other.

Meter, Current.

An instrument for measuring the quant.i.ty of electricity in current form supplied to consumers. It may be of various types. The general principle involved is that in commercial installations for incandescent light and power supply a fixed potential is usually maintained, the multiple arc system being employed. Hence all that is requisite is to measure the coulombs or the ampere-hours to know what quant.i.ty of energy has been supplied.

Meter, Electro-magnetic.

A current meter in which the current is measured by its electro-magnetic effects.

Meter-millimeter.

A unit of resistance. (See Resistance, Meter-millimeter.)

Meter, Thermal Electric.

A current meter in which the current is measured by the heat it imparts to a conductor. In one meter a very light helix of mica is poised horizontally over a conductor, and the whole is enclosed in a case. As the wire is heated it causes an ascending current of air which rotates the vane, and the latter moves delicate clockwork which moves indicating hands. The hotter the wire the more rapidly the air ascends, and consequently the speed of the vane is proportional to the current, because the heat of the conductor is proportional thereto.

Meter, Time Electric.

An electric meter which measures the length of time during which current is used. It a.s.sumes a constant current and potential. It is virtually a clock, which is turned on when the current pa.s.ses, and is turned off with the current.

Meter, Watt.

A combined current and potential meter. It is constructed on the general lines of a Siemens' Electro Dynamometer. If in it one coil is made of coa.r.s.e wire and is placed in series with the current conductor, and if the other is wound with fine wire and is connected as a shunt from point to point whose potential difference is to be determined, the instrument becomes a watt meter.

Synonym--Energy Meter.

Methven Standard or Screen.

A standard of illuminating power. It is the light emitted by a three-inch Argand gas flame through a rectangular aperture in a silver plate carried by a screen. The aperture is of such size and so far distant from the flame as to permit the pa.s.sage of exactly two candles illuminating power.

Fig. 241. METHVEN SCREEN

376 STANDARD ELECTRICAL DICTIONARY.

Mho.

A unit of conductance, not in very general use. It is the reciprocal of the ohm. Thus a resistance of ten ohms is a conductance of one-tenth mho.

Mica.

A natural mineral, a silicate of several oxides; muscovite. It is used as an insulator and dielectric. Its resistance per centimeter cube after several minutes electrification at 20? C. (68? F.) is 8.4E13 ohms (Ayrton). Its specific inductive capacity is 5, air being taken at 1.

Mica, Moulded.

An insulating material, whose body is made of mica pulverized and cemented together with heat and pressure and some suitable cement.

Sh.e.l.lac is often used as the cement.

Micro.

A prefix meaning "one-millionth of;" a micro-farad is one-millionth of a farad.

Micrometer.

An instrument for measuring small distances or small differences. It generally is based upon an accurate screw which may have a worm wheel for head, actuated by a worm or helix with graduated head, so that exceedingly small advances of the screw may be produced. The pitch of the screw being known its actual advance is known.

Micrometer, Arc.

A micrometer for measuring the distance between voltaic arc electrodes.

Micron.

A unit of length. It is one-millionth of a meter or four one-hundred-thousandths of an inch.

377 STANDARD ELECTRICAL DICTIONARY.

Microphone.

An apparatus which includes a contact of variable resistance; such resistance can be varied in amount by slight vibrations, such as those produced by sound waves. The apparatus in use forms part of a circuit including a telephone and current generator. As the contact is varied the resistance of the circuit and consequently the current intensity changes and sounds are emitted by the telephone corresponding to such changes. If the microphone is spoken to, the telephone will emit corresponding sounds, reproducing the voice.