A notable advance in the art of dynamo con struction was made by Mr. S. A. Varley in 1866 (consult his British patent of that year) and by Dr. Charles William Siemens and Mr. Charles Wheatstone (consult Society Proceed ings,' 1867), who independently discovered that when a coil of wire, or armature, of the dynamo machine is rotated between the poles (or in the of an electromagnet, a weak current is set up in the coil due to residual magnetism in the iron of the electromagnet, and that if the circuit of the armature be connected with the circuit of the electromagnet, the weak current developed in the armature increases the mag netism in the field. This further increases the magnetic lines of force in which the armature rotates, which still further increases the current in the electromagnet, thereby producing a corre sponding increase in the field magnetism, and so on, until the maximum electromotive force which the machine is capable of developing is reached. By means of this principle the dynamo machine develops its own magnetic field, thereby much increasing its efficiency and economical operation. Not by any means, however, was the dynamo electric machine perfected at the time mentioned. In 1860 an important improvement had been made by Dr. Antonio Pacinotti of Pisa who devised the first electric machine with a ring armature. This machine was first used as an electric motor, but afterward as a generator of electricity. The discovery of the principle of the reversibility of the dynamo electric ma chine (variously attributed to Walenn 1860; Pacinotti 1864; Fontaine, Gramme 1873; Deprez 1881, and others) whereby it may be used as an electric motor or as a generator of electricity has been termed one of the greatest discoveries of the 19th century. In 1872 the drum armature was devised by Heffner-Altneck. Thts machine in a modified form was subsequently known as the Siemens dynamo. These machines were presently followed by the Schuckert, Gulcher, Fern, Brush, Hochhausen, Edison and the dynamo machines of numerous other inventors.
In the early days of dynamo machine con struction the machines were mainly arranged as direct current generators, and perhaps the most important application of such machines at that time was in electro-plating, for which purpose machines of low voltage and large current strength were employed. (See ELECTRIC DIRECT CURRENT). Beginning about 1887 alternating current generators came into extensive oper ation and the commercial development of the transformer, by means of which currents of low voltage and high current strength are transformed to currents of high voltage and low current strength, and vice-versa, in time revolutionized the transmission of electric power to long distances. Likewise the introduc tion of the rotary converter (in connection with the transformer) which converts alternating currents into direct currents (and vice-versa) has effected large economies, in the operation of electric power systems. See a.m. TRIO ALTERNATING CURRENT MACHINERY.
Before the introduction of dynamo electric machines, voltaic, or primary, batteries were extensively used for electro-plating and in teleg raphy. There are two distinct types of voltaic cells, namely, the and the or type. The open type in brief is that type which operated on closed circuit becomes, after a short time, polarized; that is, gases are liberated in the cell which settle on the negative plate and establish a resistance that reduces the current strength. After a brief interval of open circuit these gases are eliminated or absorbed and the cell is again ready for operation. Closed circuit cells are those in which the gases in the cells arc absorbed as quickly as liberated and hcnce the output of the cell is practically uniform. The Leclanche and Daniell cells, re
spectively, are familiar examples of the °open" and °closed" type of voltaic cell. The °open° cells are used very extensively at present, espe cially in the dry cell form, and in annun ciator and other open circuit signal sys tems. Batteries of the Daniell or "gravity"' type were employed almost generally in the United States and Canada as the source of elec tromotive force in telegraphy before the dynamo machine became available, and still are largely used for this service or as °local" cells. Bat teries of the °gravity° and the Edison-Lalande types are still much used in °closed circuit* systems.
The possibility of obtaining the electric cur rent in large quantities, and economically, by means of dynamo electric machines gave im petus to the development of incandescent and arc lighting. Until these machines had attained a contmercial basis voltaic batteries were the only available source of current for electric lighting and power. The cost of these bat teries, however, and the difficulties of main taining them in reliable operation were pro hibitory of their use for practical lighting pur poses. The date of the employment of arc and incandescent lamps may be set at about 1877. Even in 1880, however, but little headway had been made toward the general use of these illuminants; the rapid subsequent growth of this industry is a matter of general lmowledge. (See EtEcirsic LIGHTING). The employment of storage batteries (q.v.), which were originally termed secondary batteries or accumulators, be gan about 1879. Such batteries are now utilized on a large scale as auxiliaries to the dynamo machine in electric power-houses and substa tions, in electric automobiles and in immense numbers in automobile ignition and starting systems, also in fire alarm telegraphy and other signal systems.
In 1871 the electric telegraph had grown to large proportions and was in use in every civ ilized country in the world, its lines forming a network in all directions over the surface of the land. The system most generally in use was the electromagnetic telegraph due to S. F. B. Morse of New York, or modifications of his system. (See TELEGRAPH ). Submarine cables (see CABLE) connecting the Eastern and West ern hemispheres were also in successful oper ation at that time. When, however, at the present day (1918) one views the vast applica tions of electricity to electric light, electric rail ways, electric power and other purposes (all it may be repeated made possible and practicable by the perfection of the dynamo machine), it is difficult to believe that no longer ago than 1871 the author of a book published in that year, in referring to the state of the art of ap plied electricity at that time, could have truth fully written: aThe most important and remarkable of the uses which have been made of electricity consists in its application to tele graph purposes" (Miller's (Magnetism and Electric.ty,' p. 460). The statement was, how ever, quite accurate and perhaps the time could have been carried forward to the year 1876 without material modification of the remarks. In that year the telephone (q.v.), due to Alex ander Graham Bell (q.v.), was invented, but it was not until several years thereafter that its commercial employment began in earnest. Since that time also the sister branches of electr:city just mentioned have advanced and are advanc ing with .such sigantic strides in. every direc tion that it is difficult to place a limit upon their progress. For a more adequate account of the use of electricity in the arts and industries see