<P> 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 machine (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 . </P> <P> In 1872 the drum armature was devised by Hefner - Alteneck . This machine in a modified form was subsequently known as the Siemens dynamo . These machines were presently followed by the Schuckert, Gulcher, Fein, Brush, Hochhausen, Edison and the dynamo machines of numerous other inventors . In the early days of dynamo machine construction 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 . </P> <P> Beginning about 1887 alternating current generators came into extensive operation 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 introduction of the rotary converter (in connection with the "step - down" transformer) which converts alternating currents into direct currents (and vice versa) has effected large economies in the operation of electric power systems . </P> <P> Before the introduction of dynamo electric machines, voltaic, or primary, batteries were extensively used for electro - plating and in telegraphy . There are two distinct types of voltaic cells, namely, the "open" and the "closed", or "constant", 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 are absorbed as quickly as liberated and hence the output of the cell is practically uniform . The Leclanché and Daniell cells, respectively, are familiar examples of the "open" and "closed" type of voltaic cell . The "open" cells are used very extensively at present, especially in the dry cell form, and in annunciator and other open circuit signal systems . Batteries of the Daniell or "gravity" type were employed almost generally in the United States and Canada as the source of electromotive force in telegraphy before the dynamo machine became available, and still are largely used for this service or as "local" cells . Batteries of the "gravity" and the Edison - Lalande types are still much used in "closed circuit" systems . </P>

Who contributed in developing equation that shows the relationship of electricity and magnetism