DYNAMIC THEORY 446 DYNAMO DYNAMIC THEORY, a hypothesis broached by Kant that all matter origin- ated from the action of two mutually an- tagonistic forces — attraction and repul- sion. All the predicates of these two forces are attributed by Kant to motion. As applied to heat, it is a theory or hy- pothesis — that now is generally accepted as the correct one — which represents a heated body as being simply a body the particles of which are in a state of vi- bration. This vibratory movement in- creases as the body is still more heated, and diminishes proportionately as it more or less rapidly cools. It is called also the mechanical theory of heat. DYNAMITE, an explosive produced by the admixture of nitroglycerin with a siliqeous infusorial earth known under the German name as kieselguhr. Nitro- glycerin, which can be prepared in small quantities by dropping glycerine into a mixture of strong nitric and sulphuric acids, the temperature being kept as low as possible, v/as discovered by Sobrero in 1846, but it was not till nearly 20 years later that the experiments of Alfred Nobel, who combined it with the absor- bent inert earth just mentioned, placed the new substance on a basis of practical and commercial importance. Kieselguhr is the mineral remains of a species of algae ; the stem consisted chief- ly of silica, and when the organic por- tions of the moss decayed, the tubular siliceous stem remained, retaining its shape. Beds of kieselguhr, underlying peat, are found in many countries; the principal formations in Europe being in Great Britian (especially Aberdeen- shire — that of Skye not being sufficiently absorbent) , Germany, and Norway. The raw kieselguhr, after calcination in a specially designed kiln to remove water and organic substance, is gi-ound and sifted, and finally contains about 98 per cent, pure silica with traces of lime and iron. Dynamite, which has a reddish-brown color, consists of 1 part of kieselguhr to 3 parts of nitroglycerin, and has a spe- cific gravity varying from 1.59 to 1.65. Djmamite burns with a yellowish flame, and in small quantities without danger; but explodes with great violence when fired by a detonating fuse. Dynamite is much employed in breaking up boulders and^ the heavier metal castings, also in agricultural operations for removing the roots of trees. DYNAMO, DYNAMO-ELECTRIC MACHINE, or GENERATOR, a ma- chine for transforming mechanical into electrical energy, and depending for its operation on the electro-motive force de- veloped in any conductor moved trans- versely through the lines of force in a magnetic field. The manner in which the energy transformation is effected distinguishes the dynamo from the old frictional electric machine, and detei'- mines its general plan of construction. Any dynamo must consist of at least two parts, the field-magnets which create the magnetic field, and the arma- ture which comprises the conducting system which moves relative to the field. To these may be added the commutator, a device necessary to secure uniform di- rection in the case of direct current ma- chines. Dynamos for direct current are designed invariably with moving arma- ture and fixed fields. In the case of ma- chines of comparatively small capacity, th<; field magnet may be bipolar, i. e., having one pair of poles, as in the horseshoe magnet. In fact, permanent steel magnets of this form constitute the fields of the little machines known as magneto-electric machines, or magnetos, such as are used in automobiles, in tele- phony, etc. Bipolar machines may be either of the overtype or undertype pattern, according as the space between the poles is arranged above or below the yoke, which corresponds to the bend of the horseshoe. The former arrange- ment is practicable only with smaller sizes, as it necessitates longer pedestals for supporting the bearings of the ar- mature shaft, leading to excessive vi- bration. The undertype has the dis- advantage that the field magnets must be supported clear of the iron bed-plate by brackets of non-magnetic material, such as brass or gun-metal, in order to prevent passage of the field through the bed-plate in preference to the armature. In any dynamo, the current passing through the moving armature causes a distortion and weakening of the mag- netic field. This effect becomes very apparent in bipolar machines at high loads, and the serious disadvantage may be largely avoided by increasing the number of pairs of poles in the field - magnet. Multipolar machines for con- tinuous current may have as many as twelve pairs of poles. This form of construction has, moreover, the advan- tage of enabling material to be more economically arranged, thus securing relatively lighter weight; and, in addi- tion, the speed of rotation may be re- duced in inverse proportion to the num- ber of pairs of poles for a given E.M.F. developed. These considerations have established the practice of constructing all machines for more than 150 kilo- watt output of the multipolar type, and machines of considerably smaller capa- city than this are regularly built with three or four pairs of poles.
