Fig. 239. SCHALLENBERG'S ALTERNATING CURRENT METER.
Meter, Alternating Current.
A meter for measuring alternating current, as supplied to consumers,
from an alternating current system. Like most commercial meters its only
function is the measurement of quantity; the potential difference is
maintained at a constant figure by the generating plant.
The cut shows the Schallenberg meter. It is simply an alternating
current motor (see Motor, Alternating Current), with air vanes mounted
on its spindle. A main coil passes all the current. Within this is a
second coil complete in itself, and not touching or connecting with the
other. The latter is built up of copper rings. Within the two coils, and
concentric with both is a disc of copper carried by a vertical spindle.
The same spindle carries air vanes, and is free to rotate. As it does so
it moves the indicating machinery.
The current in the outer coil induces one in the inner coil. Owing to
lag, the current in the inner one differs in phase from that in the
outer one, and a rotatory field is produced. The copper disc acquires
induced polarity, and rotates with speed which normally would be in
proportion to the square of the current. But the object of the meter is
to register the current only. The air vanes effect this. The resistance
of the air to their motion causes the rate of rotation to vary directly
as the speed.
Meter Bridge.
A form of Wheatstone's bridge in which one lateral pair of arms is
represented by a straight wire. The other pair comprise a known
resistance, and the resistance to be determined. The galvanometer is
connected on one side between the known and unknown resistance. On the
other side its connection is moved back and forth along the straight
wire until the balance is secured and the galvanometer reads zero.
The relative lengths of wire intercepted between the two ends thereof
and the movable galvanometer connection are proportional to the
resistance of these parts and give the necessary data with the one known
resistance for determining the unknown resistance.
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 passed 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 quantity 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 quantity 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 assumes a constant current and potential. It is virtually a
clock, which is turned on when the current passes, 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
coarse 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 passage 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 20º C. (68º 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.
Shellac 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.
It has been found in practice that carbon gives the best microphone
contact. One of the simplest and earliest forms is shown in the cut. A
short rod or pencil of carbon, A, such as used in batteries, is
sharpened at the ends and rests loosely in a vertical position between
two blocks of carbon, C C, in each of which a hole is drilled to receive
one of the points. The blocks are carried on a standard and base D. The
blocks are connected with two terminals x, y, of a circuit, including a
telephone and battery. There are two contacts to be disturbed.
If delicately adjusted a fly walking over the base-board will disturb
the contacts enough to produce sounds in the telephone. These sounds are
possibly not due only to sound waves, but in part to absolute mechanical
disturbances.
The various forms of telephone transmitter are generally microphones.
Fig. 242. MICROPHONE.
Microphone Relay.
A combined microphone and telephone. A microphone is placed close to the
diaphragm of a telephone. The slight sound waves emitted by the
telephone affect the microphone and are repeated in its circuit. The
microphone circuit includes a local battery and telephone.
Microtasimeter.
An apparatus for indicating minute changes in temperature or atmospheric
moisture.
378 STANDARD ELECTRICAL DICTIONARY.
A button of compressed lampblack is placed in series with a battery and
galvanometer. A strip of some substance, affected in its length either
by heat or by moisture, is held pressing against the button. A slight
change in length of the strip varies the resistance of the button and
hence affects the galvanometer. In this way exceedingly slight changes
in heat or moisture may be indicated.
For heat indications vulcanite may be used. The heat of the hand held
near it is enough to affect the galvanometer. For moisture a slip of
gelatine is used. The moisture of a damp slip of paper two or three
inches distant is sufficient to affect the galvanometer.
In the cut, Fig. 2, shows the general distribution of the apparatus in
circuit with a battery and galvanometer. C is the base of the apparatus,
from which the standard, B, with adjusting screw, H, rises. The strip of
vulcanite is held between I and G. Within D is the carbon button (F in
Fig. 3) pressed between G and E; A is a standard to carry the parts last
described. In Fig. I it is shown as part of a Wheatstone bridge, a, b
and c being resistance coils; l the tasimeter, and g the galvanometer.
If a balance is secured, any variation in the resistance of the
tasimeter will disturb the galvanometer.
Synonym--Tasimeter.
Fig. 243. MICROTASIMETER.
379 STANDARD ELECTRICAL DICTIONARY.
Mil.
A unit of length; one-thousandth part of a lineal inch.
It is equal to
.025399 millimeter;
.000083 foot;
.001000 inch.
Mil, Circular.
A unit of area; employed in designating the cross-sectional area of
wires and other circular conductors.
It is equal to
.78540 square mil;
.000507 square millimeter;
7.8E-7 (.00000078) square inch.
If the diameter of a wire is given in mils, the square of its diameter
gives its cross-sectional area in circular mils.
Mil-foot.
A unit of resistance. (See Resistance, Mil-foot, Unit of).
Mil, Square.
A unit of area; one-millionth of a square inch.
It is equal to
.000645 square millimeter;
1.2733 circular mil;
.000001 square inch.
Milli.
A prefix; one-thousandth. Thus a milligram is one-thousandth of a gram;
a millimeter is one thousandth of a meter.
Milligram.
A unit of weight ; one-thousandth of a gram, q. v.
It is equal to
.015432 grain;
.000032 troy ounce.
Millimeter.
A unit of length; one-thousandth of a meter.
It is equal to
39.37079 mils;
.03937 inch;
.00109 yard.
380 STANDARD ELECTRICAL DICTIONARY.
Milli-oerstedt.
A proposed but not adopted unit of current; one-thousandth of an
oerstedt. It is equal to one-thousandth of an ampere.
[Transcriber's note: oersted: 1. CGS unit of magnetic intensity, equal
to the magnetic pole of unit strength when undergoing a force of one
dyne in a vacuum. 2. Formerly, the unit of magnetic reluctance equal to
the reluctance of a centimeter cube of vacuum between parallel
surfaces.]
mm.
Contraction for millimeters.
Molar.
Referring to phenomena of mass as gravitation. Mechanics
generally treats of molar laws and phenomena.
[Transcriber's note: Molar, or mole, often refers to a quantity of a
substance containing an Avagadro number (6.02E23) of molecules--a weight
equal to the atomic weight of the molecule. For example, a mole of
hydrogen (H2) is 2.015 grams; sodium chloride (NaCl) is 58.443 grams.]
Molar Attraction.
The attraction of mass for mass; gravitation. Synonyms--Mass
Attraction--Gravitation.
Molecular Affinity.
The attraction of molecules for each other as seen in the formation of
double salts, the combining of water of crystallization with a salt, and
in other cases; a phase of affinity belonging to chemistry, although
outside of true atomic attraction.
Molecular Attraction.
The attraction of molecules; physical affinity. Cohesion, the attraction
of similar molecules for each other, and adhesion, that of dissimilar
molecules, are examples. This should be distinguished from molecular
affinity, a phase of chemical force.
Molecular Bombardment.
When a gas contained in a vessel is brought to a sufficient state of
rarefaction the molecules cease to be subject to the laws of diffusion,
but move back and forth in straight lines from side to side of the
vessel. Their courses can be affected by electric discharge, which can
cause them to all impinge upon one of the electrodes, the positive one,
producing luminous effects. The path, if referred to the negative
electrode, tends to be normal to its surface, so that the resultant path
may be curved, as the stream of molecules go to the positive electrode.
The fanciful name of molecular bombardment is given to the phenomenon,
the luminous effect being attributed to the impinging of the molecules
against the positive electrode as they are projected from the positive.
The course of the molecules is comparable to the stream of carbon
particles from the positive to the negative electrode in an arc lamp.
(See Matter, Radiant.)
Molecular Chain.
The theoretical rows of molecules supposed to extend from anode to
cathode in an electrolytic cell (see Cell, Electric--Grothüss'
Hypothesis) are called molecular chains.
381 STANDARD ELECTRICAL DICTIONARY.
Molecular Rigidity.
The tendency of the molecules of a mass to retain their position in a
mass in resistance to polarizing or depolarizing force, the first being
the effect of a magnetic field. It is the theoretical cause of coercive
force, q. v., and of residual magnetism. (See Magnetism, Residual.)
Molecule.
The smallest particle of matter that can exist alone. It is made up of
atoms, but an atom can never exist alone, but only, with one or two
possible exceptions, combined with one or more other atoms as a
molecule. The molecules under present conditions are not in constant
contact with each other, but are perpetually vibrating through paths, in
solids probably in defined paths, in liquids and gases in perpetually
new paths. The molecules collide with each other and rebound. This
motion is the kinetic motion termed heat. At the absolute zero--minus
273.72° C. (-460.7° F.) the molecules would be in contact and quiescent.
In the gaseous state the molecules of most substances occupy the same
volume; those of a few elements occupy one-half and of others twice the
normal volume. The mean free path of the molecule of hydrogen is about
1/20,000 mm. (1/508,000 inch) (Maxwell) or twice this length (Crookes),
the collisions in hydrogen are about 17,750 millions per second; the
diameter is about 8/10,000,000 mm. (8/254,000,000 inch) ; A particle of
matter 1/4,000 mm. (1/102,000 inch) contains, it is supposed, about
40,000 molecules. The results of different authorities vary so widely as
to deprive the subject of much of its interest. A Sprengel pump, such as
used for exhausting Geissler tubes, or incandescent lamp bulbs, may
leave only one hundred-millionth (1/100,000,000,) of an atmosphere
present, giving the molecules a capability of an average free path of
vibration 33 feet long.
Moment.
When a force is applied so as to tend to produce rotation around a
point, the product of the force by the shortest distance from the point
of rotation to the extension of the line of the force. Such distance is
the perpendicular to the extension of the line through the point of
rotation.
Mordey Effect.
A phenomenon observed in dynamo armatures. At full loads the hysteresis
decreases. The effect is thus expressed by S. P. Thompson. "When an
armature core is rotated in a strong magnetic field, the magnetization
of the iron is being continually carried through a cycle, but in a
manner quite different from that in which it is carried when the
magnetizing force is periodically reversed, as in the core of a
transformer. Mordey has found the losses by hysteresis to be somewhat
smaller in the former case than in the latter."
Morse Receiver.
The receiving instrument formerly universally used in the Morse system.
It is now but little employed, the sounder having displaced it. Several
types were invented.
It consists of machinery which carries a reel of paper ribbon arranged
to be fed over a roller by clockwork. A pencil, inking roller, or
embossing stylus (for the latter the roller must have a groove) is
carried by an arm with restricted range of vibration just over the paper
and roller. The armature of an electro-magnet is attached to the arm.
When the magnet is excited the armature is attracted and the marking
device is pressed on the paper. If the clockwork is in operation the
marker will make a line as long as the armature is attracted. When
released no mark will be produced. In this way the dots and dashes of
the Morse code are made on a ribbon of paper.
As an inking arrangement a small roller is carried by the end of the
vibrating arm. The embosser, or dry point stylus, was very extensively
used. The clockwork was generally driven by descending weights.
Synonym--Morse Recorder.
382 STANDARD ELECTRICAL DICTIONARY.
Mortar, Electric.
An electric toy which may have various modifications. In the cut a
wooden mortar with recess to receive a ball is shown. Two wires enter
the base but do not touch. On placing the ball in position and passing a
spark from a Leyden jar across the interval between the wires, the heat
and disturbance are enough to project the ball. Gunpowder may be used,
the discharge being passed through a wet string to prolong the spark.