Norwich bulletin., 1922-06-17
MICA AM EssENTIA. IN ELEcTRcL INDUSTRY.


 Mica, that odd. heat-resisting. trans parent mineral which did duty as win dows in the heating stoves of our boy hood days, has since now become so es- sentiaI in electrical industry, that the larger electrical-supply manufacturing companies own and operate their own mines according to the United States bu- reau of mines. Mica includes a group of several minerals characterized by a perfect basal clevage, by virtue of which they may be split in all exceedingly thin rlates. The minerals are of very com mon occurrence in all countries, consti- tufing about per cent. of the igneous rocks of the world. The term "isiil- mica; but this is incorrect, for true inis- glass Is double combustib1e material made from the air bladders of certain fish
 Silvery flakes of white mica and vel low flakes of altered black mica are commonly found in rocks and are often mistaken for the precious metals. Any observer may easily distinguish mica from gold or silver, by testing with the blade of 8 sharp penknife. The mica will split off in thin, flexible flakes and pulyerize to a powder, while gold or silver have no cleavage, and are malle- able. The mica found as scattered flakes in various types of rock is of no com mercia1 value. Mica used in commerce Is obtained from relatively few deposits where 1t appears in sheets fairly free from defects, and having an area of at least several square inches:
 There are several varieties of mica, such as muscovite (lznoTn as potash or white mica), phlogopite or amber mica, bictite or balack mica, paragonite or so- dfum mica, lepidolite or ,lithiun mica, 1epidomelane or iron mict,. roscoelite or vanadium mica, and others. Muscovite and phlogopite are the only ones that are of importance commercially.
 Mica possesses combination of spe- ciaI qualities, the chief of which are elasticity, toughness, sfexibility, trans parency, ability to withstand excesseve heat and sudden changes in tenpera- ture, high dielectric strength, cleavabili- | ty into thin films, and resistance to de composition. No other substance pos sesses this combination of qualities, con sequently no satisfactory substitutes fol mica are known for the more important uses. The brilliancy of the reaecting surfaces give mica special value for dec orative purposes Muscovite in com mercial sizes occurs in many parts O 1 the world Mica is found in masse! i known as blocks" varying in widtf l and length from a few inches to sev 1 eral feet, and in thickness from a frac l tion of an inch to inches or more. Rum ruby or green shades of color are mos l in demand.
 In 8 recent year Canada. the Unitet 1 States and India produced about 97 re cent of the world output of sheet mica Of this amount India produces abou 65 per cent and the United States 1 per cent The output of India and o the United States Is entirely musco I vite, whereas. the Canadian productio is mostly entirely phlogorite. The pre- war yearly world consumption of shee mica was estimated at $500 short ton: I Of this, the United States consumed ap
 mica producing states in order of pro
 auction were as follows: North DalSc ta Virginia New Mexico Georgia, Sout Dakota, California and Wisconsin. It
 ability to withstand heat and it's hig electrical resistance has led to wide ar
 pIication of sheet mica in the electrics
 industries. An important use of cleo
 trical mica is for inter-leaving betwee
 the copper segments of commutators
 Thin films are used in vast numbers
 condensers for magnetos and wireles
 apparatus. As sheets in greatly diver sified shapes, or as washers and tube mica is used extensively as an insuls tor In dynamos and in various fittings ( appliances, in fuse boxes, sockets, in sulators, electric heaters, f1atirons, tel phones, etc As heat-resisting tram parent medium sheet mica has variol uses. It was formerly widely ernplo! ed for stove windows, but this use hs
 declined to a considerabl extent. Mic