ELECTRICITY DURING THE NINETEENTH CENTURY. 337 



electric illumination. Accordingly^ we find that suggestions were soon 

 made for electric-arc lamps to be operated by these more powerful and 

 constant sources of electric current. The first example of a working 

 type of an arc lamp was that brought to notice by W. E. Staite, in 

 1847, and his description of the lamp and the conditions under which 

 it could be worked is a remarkably exact and full statement, consid- 

 ering the time of its appearance. Staite even anticipated the most 

 recent phase of development in arc lighting, namely, the inclosure of 

 the light in a pai'tially air-tight globe to prevent too rapid waste of 

 the carbons by combustion in the air. In a public address at Newcastle- 

 on-Tyne, in 184:7. he advocated the use of the arc so inclosed in 

 mines, as obviating the danger of fire. But it was a long time before 

 the electric arc acquired anj^ importance as a practical illuminant. 

 There was, indeed, no hope of its success so long as the current had 

 to be obtained from batteries consuming chemicals and zinc. The 

 expense was too great and the batteries soon became exhausted. In 

 spite of this fact, occasional exhibitions of arc lighting were made, 

 notably in 1856 by Lacassagne and Thiers in the streets of Paris. 



For this service they had invented an arc lamp involving what is 

 known as the differential principle, afterwards applied so extensively 

 to arc lamps. The length of the arc or the distance between the car- 

 bons of the lamp was controlled with great nicety, and the light thus 

 rendered ver}^ steady. Even as late as 1875 batteries were occasionally 

 used to work single electric arc lamps for public exhibitions, or for 

 demonstration purposes in the scientific departments of schools. The 

 discovery of the means of efficiently generating electricity from 

 mechanical power constitutes, however, the keynote of all the won- 

 derful electrical work of the closing years of the nineteenth century. 

 It made electrical energy available at low cost. Michael Faraday, a 

 most worthy successor of Davy at the Royal Institution, in studying 

 the relations between electric currents and magnets, made the exceed- 

 ingly important observation that a wire, if moved in the field of a 

 magnet, would yield a current of electricity. Simple as the discovery 

 was, its effect has been stupendous. Following his science for its own 

 sake, he unwittingly opened up possibilities of the greatest practical 

 moment. The fundamental principle of the future dynamo-electric 

 niachine was discovered by him. This was in 1831. Faraday's mves- 

 tigations were so complete and his deductions so masterly that little 

 was left to be done by others. Electro-magnetism was supplemented 

 by magneto-electricity. Both the electric motor and the dynamo gen- 

 erator were now potentially present with us. Faraday contented him- 

 self with pointing the way, leaving the technical engineer to foUow. 

 In one of Faraday's experiments a copper disk, mounted on an axis 

 passing through its center, was revolved between the poles of a large 



