THE DEVELOPMENT OF ELECTRICAL SCIENCE. 229 



probably it comes in well here. Just how to apply Maxwell's equation 

 to the Eontgen rays is not yet quite clear, but there is no doubt as to 

 the great importance of the discovery. 



As an outcome of all this activity in the determination of standards 

 and in the absolute measurements of the electrical properties of mate- 

 rials, combined with the great commercial demand produced by the 

 introduction of dynamo machinery, we have now many excellent instru- 

 ments at our disposal for absolute measurement and suitable either for 

 practical applications or for the most refined laboratory work. For the 

 production of these we are indebted to a host of inventors, prominent 

 among whom may be mentioned Lord Kelvin, Lord Eayleigh, Ayrton 

 and Perry, Mather, Swinburne, Cardew, and Westou. 



Magneto-electric and dynamo- electric generators and motors have 

 now become so common that we are apt to forget that their introduc- 

 tion on an extensive scale has only taken a few years. Faraday's disk 

 dynamo was, as has already been stated, produced in 1831, and a 

 machine for generating electricity was made by Pixii in the following 

 year. Pixii's machine consisted of a horseshoe permanent magnet 

 which was rotated in such a way that its poles passed alternately in 

 front of the poles of a similar electro- magnet. An alternating current 

 was thus induced in the circuit which included the coils of the electro- 

 magnet. 



This machine was improved by Clarke, who rotated the coils and put 

 a commutator on the axis. Other machines were made or suggested 

 by various physicists, and an important observation, which has since 

 been frequently overlooked, was made at this time by Jacobi, who 

 pointed out the importance of making the cores of the coils short. 

 Sturgeon, in 1835, made a dynamo with a shuttle-shaped armature; a 

 similar form has long been identified with the name of Siemens. Wool- 

 rich made a multipolar-magneto machine in 1841 for electroplating, and 

 Wheatstone about this time produced his small multipolar magneto, 

 long used for telegraph purposes. In 1845 Wheatstone and Cooke 

 patented the use of electro-magnets in place of the permanent magnets, 

 and Brett suggested, in 1848, that the current from the machine might 

 be made to pass round a coil surrounding the magnet and thus increase 

 its strength. A similar suggestion was independently made in 1851 

 by Sinsteden. In 1849 Pulvermacher proposed the use of thin lamina; 

 of iron for the cores of the magnet, a proposition which has since, but 

 probably for a different reason, been almost universally adopted. Sin 

 steden used iron wire cores and made a number of experiments on the 

 effect of varying the pole face. About this time another class of 

 machines were proposed by Eitchie, Page, and Dujardin. In these 

 machines both the magnets and the coils were to be stationary, but 

 the magnetism was to be varied by revolving soft iron pieces in front 

 of the poles. Modern representatives of these machines are to be found 

 in the dynamos of Kingdon, Stanley, and others. All the machines up 



