13 



owing to this quality it becomes seriously heated during com- 

 pression. 



Electricity. The cheapness with which electricity can be 

 generated will doubtless shortly render this an agent commercially 

 suitable for transmitting energy ; for owing to the invention of 

 the dynamo, which is a machine which turns mechanical energy 

 into electricity, and which, by reversion, converts electricity into 

 mechanical energy, very little energy is wasted. The principle 

 upon which this conversion depends, is a movement of coils of 

 copper- wire in a magnetic field : when a coU is moved in a mag- 

 netic field a current is established, and this can be made available 

 for the conveyance of electrical energy to a distance. The usual 

 source of energy for driving a dynamo is coal or gas. The wires 

 which pass from the dynamo are distributed on either the parallel 

 or the continuous system. In the parallel system there are two 

 wires connecting, and wires pass from one to the other through 

 lamps or motors as may be required. In this case a large quantity 

 of electricity with a low electro-motive force is wanted. In the 

 continuous system a continuous wire is passed through a series of 

 lamps and motors, and in this case a smaller quantity of electricity 

 with a higher force is necessary. Electric currents can also be 

 derived from a battery, from an accumulator which is a form of 

 battery, and from heat. A^Tien a current passes along a Avire, the 

 wire becomes heated to an extent which depends upon the resistance 

 to the passage of the electricity. In the case of large currents the 

 heating becomes very serious indeed. This is diminished by driving 

 the machine at a high speed. If we are to have a large quantity of 

 work done we must use a high electro-motive force, as we thus get 

 the greatest efficiency. But there is danger to life in these high 

 electrical pressures, and it is here that the use of secondary 

 batteries, or accumulators, comes in. An accumulator is a ceU con- 

 sisting of lead plates which have been coated with oxide of lead. 

 A current of electricity is passed through the cell, the electrical 

 energy in motion is converted into chemical energy at rest, and 

 this remains until it is called forth when wanted to be transformed 

 into electrical energy. 



The chief cause of loss of power in the working of all machines 

 is friction of the bearings or of other parts. 



