DYNAMO-ELECTRIC MACHINE             569                                        DYNAMOMETER

(see Transformers and Transmission of Power), and even for many shorter lines there are engineering advantages in its use. The field-magnets of some of the earlier dynamos were permanent steel magnets, and such machines were called magneto-electric machines or magnetos. The little dynamos used in telephone call-boxes are magnetos. The field-magnets of all modern machines are electro-magnets. The machine is spoken of as bipolar or multipolar, according as the field-magnet system has two or more poles. The field-magnets of most alternators are separately excited by an electric current from a small auxiliary dynamo called the exciter. The field-magnets of most D. C. dynamos are excited by the current generated by the dynamo itself. If all the current of the armature passes through the field-coils, the dynamo is said to be a series-wound machine; if only a fraction of the armature-current goes through the field-coil, the dynamo is a shunt-wound machine. . When there are two sets of field-coils, a series-set and a shunt-set, the dynamo is said to be compound-wound. The choice of the kind of field-windings is a question of regulation of the current-output with different loads.

Dynamos are rated in kilowatts as steam-engines are rated in horse-power. A kilowatt (K. W.) is almost exactly equal to one and one third H. P. Thus a 75 K. W. dynamo has the same work-capacity as a

100 H. P. steam-engine. To get the K. W. of a dynamo, divide the product of the volts and amperes by 1,000. The use of electrical machinery for power-purposes has made a demand for large dynamos, often reaching several thousand K. W. for one machine. Thus in the Niagara Falls plant there are ten machines, each of 5,000 K. W. output.

Faraday and others saw immediately that his discovery of electromagnetic induction (1831) could be used for the con-

tinuous production of electric currents, but the modern machine was evolved slowly and represents the added ideas of more than a generation of inventors. The first machine had permanent magnets. In 1832 Pixii devised a commutator. In 1848 Brett suggested the self-exciting electromagnet for the field. The name dynamo-electric machine was first given by Dr. Werner Siemens in a paper read before the Berlin academy in 1867. In 1870 Gramme invented the ring-armature, and three years later Hefner-Alteneck improved the early form of Siemens ' armature and gave us the present drum-armature. The ring- and drum-armatures have been used more than any others in D. C. dynamos. The theory of the dynamo-machine has been studied carefully by Hopkinson, Frölich, Kapp, S. P. Thompson and others, so that the designing of a dynamo for any specific purpose has been reduced to a question of exact calculation. The efficiency of modern dynamos is often 96 to 98 per cent. The largest makers of dynamo-electric machinery in the United States are the General Electric Company, the Westinghouse Electric and Manufacturing Co. and the Stanley Electric Manufacturing Co. The shops of these companies are among the largest manufacturing-plants of any kind in the world.

A. P. Carman. Dy'namom'eter, strictly an instrument to measure a force tending to produce motion. Thus, a spring-balance is sometimes called a dynamometer. The term, however, is commonly applied to an instrument to measure power. Power is rate of doing work. Work is force overcome through distance. The unit of work used in' engineering "is the work done in raising a pound-weight one foot against gravity, called a foot-pound. When a machine does work at the rate of 33,000 foot-pounds in one minute, the power is one horse-power (H. P.). The power-unit used by electricians is a kilowatt (K. W.), equal to ij H. P. A dynamometer is technically used in engineering for any device which measues H. P. One of the simplest and most common forms is the Prony brake. A block B is clamped fairly tight on the pulley O of the machine. The machine then does work against friction. The force is balanced by the weight Won the lever-arm AO. From the speed of rotation, the lever-arm and the force W, the power is directly calculated. For the many forms of dynamometers, see J. J. Flather's text-book on Dynamometers. A wattmeter measures electrical power in watts. It is an instrument for measuring the product of the

direct connected dynamo and engine