But it was a long time before the electric arc acquired any 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 carbons of the lamp was controlled with great nicety, and the light thus rendered very 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 key-note of all the wonderful 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 exceedingly 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 machine was discovered by him. This was in 1831. Faraday’s investigations 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 generator were now potentially present with us. Faraday contented himself with pointing the way, leaving the technical engineer to follow. In one of Faraday’s experiments a copper disk mounted on an axis passing through its centre was revolved between the poles of a large steel magnet. A wire touched the periphery of the disk at a selected position with respect to the magnet, and another was in connection with the axis. These wires were united through a galvanometer or instrument for detecting electric current. A current was noted as present in the circuit so long as the disk was turned. Here, then, was the embryo dynamo. The century closed with single dynamo machines of over 5000 horse-power capacity, and with single power stations in which the total electric generation by such machines is 75,000 to 100,000 horse-power. So perfect is the modern dynamo that out of 1000 horse-power expended in driving it, 950 or more may be delivered to the electric line as electric energy. The electric motor, now so common, is a machine like