DYNAMO-METAMORPHISM 



DYSART 



velocity. This does not hoJd good 

 of the " induction " alternator in 

 which the frequency depends upon 

 the characteristics of an external 

 (polyphase) alternating current 

 producing the field. So far, alter- 

 nators have only been considered 

 in respect of what is known as a 

 " single-phase " circuit. If a second 

 set of armature coils be inter- 

 posed in the spaces between the 

 original set (doubling the arma- 

 ture, in fact) two distinct alternat- 

 ing currents will be generated, the 



I 



Dynamo. Fis 13 1, 000 k.w. \7est- 



ingbpuse 3-pbase alternator, showing 



field magnets inside armature 



one following the other at a quarter 

 of a period. For this reason this is 

 sometimes called the " quarter- 

 phase " system. If the principle 

 be extended to three similar, but 

 separate sets of coils, three equal 

 alternating currents, one-third of a 

 phase apart, will be obtained (see 

 Fig. 14). The use of " polyphase " 

 currents secures a greater output 

 for given weight of generator and 

 al?o requires less copper in trans- 

 mission lines and is far more suit- 

 able tor motor circuits. See 

 Distributor. 



Dynamo -Met amor phism (Or. 

 mela. implying change ; morphe, 

 form). Alteration of rock-struc- 

 ture by the lateral pressures in- 

 duced by movements in the earth's 

 crust. The term was introduced 

 by A. Harker to denote the effects 

 of high pressure and low tempera- 

 ture, therm o-metamorphism being 

 used to denote the effects of low 

 pressure and high temperature. 

 The alternative terms, regional and 

 contact metamorphism, are roughly 

 synonymous with dynamic and 

 thermal metamorphism respec- 

 tively. 



The changes produced are phy- 

 sical and mineralogical. and usually 

 render rooks more highly crystal- 

 line. Homogeneous rocks under 

 pressure develop cleavage-planes, 

 foliation and schistose structure. 

 Thus clays, shales, or fine-grained 

 volcanic dust may become roofing- 

 slates, coarse-grained rocks may 

 become gneisses. Heterogeneous 

 rocks, if brittle and yielding strata 

 are intermingled, develop folding 

 and faulting. Mineralogical 



Uyuamo. L'ig. 14. Armature of 

 3-phase alternator, 6,600 voits 



changes include re -crystallisation 

 into a mosaic of smaller crystals 

 and the formation of mica. Sea 

 Crystallography. 



Dynamometer (Gr. dynamis, 

 power ; metron, a measure). De- 

 vice for measuring force or power. 



Though the term dynamometer 

 has been extensively used for many 

 different kinds of measuring in- 

 struments, it is more commonly 

 applied to instruments used for 

 measuring the h.p. of engines. 

 They may be divided into three 

 classes: (1 ) those for measuring the 

 pull of anything ; (2) those for 

 measuring the push or thrust ; and 

 (3) those for measuring twisting 

 power or torsion. 



The first type measures such 

 forces as those exerted by railway 

 locomotives, traction engines, etc., 

 and consist essentially of a power- 

 ful spring balance through which 

 the power is applied. The second 

 type measures such forces as the 

 t h r u s t of an _ 

 aeroplane pro- 

 peller or 

 ste am s hip 

 screw, and the IHf 

 third the force 

 exerted by a 

 revolving 

 shaft, and I 

 both the latter \ iLw^ 

 may consist ; 

 of recording ! Twifa 

 springs or 

 brake attach- 

 ments. 



When a 

 brake is used 

 it absorbs 

 power and the 

 dynamometer 

 is called an 

 absorption dy- 

 namometer. 

 Transmis- 

 sion dyna- Dynamometex. Com- 

 mometers mon type of brake 

 measure the dynamometer 

 horse-power of machines without 

 any absorption of power, save that 

 due to friction, and the majority 

 consist of recording spring devices. 





The illustration shows a counon 

 type of brake dynamometer. To 

 one end of a rope," encircling the fly- 

 wheel of the engine, is attached a 

 weight, and the other end is fast- 

 ened to a spring. The motion of the 

 wheel tends to lift the weight, and 

 this tendency is measured on the 

 spring and from it, and the known 

 revolutions per minute of the fly- 

 wheel, the horse-power being 

 exerted may readily be calculated. 



Dynamotor. Combined form of 

 motor and dynamo. It consists of 

 one field magnet and two arma- 

 tures, or one armature with two 

 windings, one receiving current as a 

 motor and the other generating 

 current as a djmanio. 



Dyne (Or. dynamis, power). Unit 

 of force which, applied to a mass of 

 one gramme, produces an accelera- 

 tion of one centimetre per second 

 every second. This force is very 

 nearly equal to the force with 

 which the earth attracts a weight 

 of one milligramme. The erg, the 

 unit of work, is done when a body 

 acted on by a force of one dyne 

 moves through a centimetre in the 

 direction of the force. Approxi- 

 mately an erg of work is done 

 when a milligramme is raised 

 through one centimetre. 10,000,000 

 ergs equal one joule. 



Dynobel. British safety ex- 

 plosive of the perchlorate type. It 

 consists of nitroglycerine, 32 '5 p.c., 

 partially gelatinised with nitro- 

 cellulose, 0'7 p.c., potassium per- 

 chlorate, 27 p.c., wood meal, 10'3 

 p.c., and ammonium oxalate. 29 '5 

 p.c. The dry salts and part of the 

 wood meal are placed together in a 

 pan and mixed by hand ; the 

 partially gelatinised nitroglycerine 

 is then added, the container being 

 wiped out with the rest of the 

 wood meal, and the whole roughly 

 mixed by hand and then incor- 

 porated in a machine of the type 

 employed for blasting gelatine or 

 cordite at a temperature of about 

 30 C. for an hour. 



Dysart (Lat. desertitm, solitude). 

 Royal and tnun. burgh and seaport 

 of Fifeshire, Scotland. It stands 

 on the Firth of Forth, 28 m. N.E. 

 of Edinburgh by the N.B. Rly. It 

 derives its name from a cave' near 

 Dvsart House reputed to have 

 been the cell of S. Serf. It has a 

 good harbour, and engages in 

 linen manufacture, shipbuilding, 

 and the export of coal. Pop. of 

 mun. bor. 4,197. 



Dysart, EARL OP. Scottish title 

 borne since ] 643 by the families of 

 Murray and Tollemache. Sir Lionel 

 Tollemache, a member of a family 

 long settled in Cheshire, was made 

 a baronet in 1611. His grandson, 

 Lionel, married Elizabeth, daugh- 

 ter of William Murray, who, in 



