DYNAMO-ELECTRIC MACHINERY. 571 DYNE. complete revolution <liiiiiif; one period. If tlien an arnialuie were niounteil within the rin^ and this armature were provided with a winding which was short-eireuited upon itself, it is evi- dent that the movins; majxnetie field would in- FlG. IT. duce currents in this winding. These currents would in turn react upon the field of force and tend to move across it. that is, the armature would be carried around with the magnetic field and the device would become an induction motor. It will be obvious that three-phase currents could lie used as well as two-phase currents. Single- phase induction motors are also made in small sizes, but the principle according to which they operate is somewhat more intricate than that governing polyphase motors. Polyphase-induc- tion motors are capable of being used for prac- tically all the purposes to which series or shunt wound direct-current motors are applied. They are simple in construction, have no moving con- tacts, and do not easily get out of order. They have been extensively employed for power dis- tribution in mills and factories, and in Europe they are used to propel electric-railway cars. The characteristics of the induction motor vary greatly according to the way in which it is con- structed. It may be built to have great starting torque, and in other ways possess the valuable qualities of the direct-current series-wound mo- tor, or it may be so constructed that it will run at nearly constant speed over a wide range of load, thus resembling a shunt-wound direct-cur- rent motor. In conclusion, it may not be amiss, since al- ternating-current apparatus is so extensively in use, to discuss briclly some of its general char- acteri-tics and devices for consideration. For long-distance power transmission alternating cur rents are used almost exclusively. This is due to the fact that a large alternating current of low potential may be converted into a small oir- rent of high potential by a simple device known as a transformer (see TR.wsFOR.NfER.s) . and by this means a great saving in the copper reipiired in the transmission line may be accomplished. At the distant end of the line another trans- formation may be used to 'step down' or re- convert the current to one of low potential. For long-distance transmission work three-phase currents are peculiarly adapted, on account of the fact that a three-phase current of a given Vol. VI.— 87 potential may be transmitted with only three- fourths of the copper required for a single-phase, two-phase, or direct current working at the same voltage. See Power, Transmi.ssion of. Among the many books available to the stu- dent of dynamo-electric machinery, the follow- ing will be found helpful : For tlie general reader, Thomi)son. DynuinuEleclric Machinery (Xew York, ISHO): Sheldon. UynamoElcctric Machinery : vol. i.. Direct Current Machines (Xew Vork. 1001) : vol. ii.. .Ulernaling Current Machines (New York, 1902); Crocker, Electric lAyhlinfi (Xew York, 1896). For the engineer: Wiener, Practical Calculation of Dynamo-Elec- tric Machines (Xew York. 1898): Jackson, Elcctro-Ma(i}ietisni and the Conslrin-lion of Dynamo and Alternating Current Machinery (Xew York. 1893) : Foster. Electrical Engineers Pocket Hook- (Xew York. 1901: Steinmetz, Ele- jnents of Theoretical Electrical Engineering (New York, 1900). For current developments in djTia- mo-electric machinery, the engineering papers and the printed matter of the electrical manufactur- ing companies should be consulted. DY NAMOM'ETEB (from Gk. divafiis'. dy- iiamis. i)uM-r -r /j^Tpov, inetron, measure >. A device for measuring the force exerted in over- coming resistance and producing n'lotion. The foot-pound, as a unit of work, has for its factors the force acting and the distance through which it acts. The horse-power, besides these factors, has a third — the time during which the force is exerted. Hence, in getting the data from which the work of a machine is to be calculated, we are to observe the force, the distance, and the time required to accomplish a certain result. Strictly speaking, the dynamometer indicates the first of these items, but it may be so arranged as to show both the others. Dynamometers are designed to indicate the force of traction, of thrust, or of rotation. A traction dynamometer may be interposed, for example, between a team of horses and a reaper or a plow, to measure the force exerted by the horses in drawing the machine. It is usually some sort of spring bal- ance, fitted with an index and a scale: the figures on the scale show the number of pounds re- quired to bring the index to the corresponding points, if the instrument were hung up and weights suspended by it. A dynamometer for thrust can lie connected with the screw shaft of a steamship to measure the force with which the screw is driving the vessel through the water. Rotary dynamometers measure the force of i shaft citlier by showing what force is required to hold the shaft in check, by absorbing the mo- tion, or by ascertaining the force the shaft tran-niit-i to other machinery. DY NAMO'TOR. Sec Dyx.mio Ki.Kt tric M.v- cuiyr.KY. DYNE, din (abbreviated from dynnm. from Ck. ovva/jut, dynamis. power) . The unit of force of the C. (t. S. system — that is, it is such a force that under its action a body of mass 1 gram is given an acceleration unity. Bodies of all masses fall toward the earth with the same acceleration at any one place on the earth's sur- face, provided they are allowed to fall perfectly freely. (See (Gravitation.) This acceleration for all places is about 980 on the C. G. S. sys- tem: therefore, the neight of a body whose mass is m grams — i.e. the force of the earth actinfj
