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 b.u.t.ton 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 b.u.t.ton. A slight change in length of the strip varies the resistance of the b.u.t.ton 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 b.u.t.ton (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 ma.s.s as gravitation. Mechanics generally treats of molar laws and phenomena.
[Transcriber"s note: Molar, or mole, often refers to a quant.i.ty 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 ma.s.s for ma.s.s; gravitation. Synonyms--Ma.s.s Attraction--Gravitation.