An ingenious device places an electric heater inside a hot water bag, thus keeping it at a uniform temperature for sick-room and hospital use.

CHAPTER VIII.

Electric power.

On the discovery of electromagnetism (Chap.  IV.), Faraday, Barlow, and others devised experimental apparatus for producing rotary motion from the electric current, and in 1831, Joseph Henry, the famous American electrician, invented a small electromagnetic engine or motor.  These early machines were actuated by the current from a voltaic battery, but in the middle of the century Jacobi found that a dynamo-electric generator can also work as a motor, and that by coupling two dynamos in circuit—­one as a generator, the other as a motor—­it was possible to transmit mechanical power to any distance by means of electricity.  Figure 76 is a diagram of a simple circuit for the transmission of power, where D is the technical symbol for a dynamo as a generator, having its poles (+ and -) connected by wire to the poles of M, the distant dynamo, as a motor.  The generator D is driven by mechanical energy from any convenient source, and transforms it into electric energy, which flows through the circuit in the direction of the arrows, and, in traversing the motor M, is re-transformed into mechanical energy.  There is, of course, a certain waste of energy in the process, but with good machines and conductors, it is not more than 10 to 25 per cent., or the “efficiency” of the installation is from 75 to 90 per cent—­that is to say, for every 100 horse-power put into the generator, from 75 to 90 horse-power are given out again by the motor.

It was not until 1870, when Gramme had improved the dynamo, that power was practically transmitted in this way, and applied to pumping water, and other work.  Since then great progress has been made, and electricity is now recognised, not only as a rival of steam, but as the best means of distributing steam, wind, water, or any other power to a distance, and bringing it to bear on the proper point.

The first electric railway, or, rather, tramway, was built by Dr. Werner von Siemens at Berlin in 1879, and was soon followed by many others.  The wheels of the car were driven by an electric motor drawing its electricity from the rails, which were insulated from the ground, and being connected to the generator, served as conductors.  It was found very difficult to insulate the rails, and keep the electricity from leaking to the ground, however, and at the Pans Electrical Exhibition of 1881, von Siemens made a short tramway in which the current was drawn from a bare copper conductor running on poles, like a telegraph wire, along the line.