27 



that by these means a more powerful current is produced. 



This was discovered by Faraday, and he also discovered that the 

 induced current of Electricity is increased or decreased according 

 to the number of lines of force that were cut. In this diagram the 

 parallel lines represent lines of force. A coil of wire is made to 

 move through the magnetic field without cutting any lines of force ; 

 no current whatever is generated in the wire. Neither will there 

 be any if the coil be so moved in the field as to cut the same num- 

 ber of lines of force. But if it be so moved as to alter the number 

 of these lines which it cuts as in these two diagrams, a current of 

 Electricity will be started, and the greater the difference between 

 the number which it cuts, the greater will be the current. This 

 principle is of great importance in the construction of the Dynamo. 



In this diagram we have a bobbin of wire moving round in a 

 circle from the North pole to the South pole. The dotted lines 

 represent lines of force. It is very evident that all the time the 

 coil is moving round in a circle, it is not at any two moments 

 cutting the same number of lines of force ; consequently all the 

 time it is moving, a succession of momentary currents rapidly 

 foUow each other round the wire, and can be collected. 



With these preliminary remarks we now come to the Dynamo 

 itself. 



These upright blocks of iron, carefully joined at the top, and 

 thickly covered with wire are called the Field Magnets, one being 

 south the other north. They are also shewn plainly in these 

 diagrams. When the Dynamo was in its infancy, the fields were 

 made of permanent steel magnets. It has been found however, 

 that if temporary soft iron magnets be used instead, the efficiency 

 of the machine is greatly increased, chiefly from the fact than an 

 electro-magnet can be made much more powerful than a per- 

 manent one of the same size. The ends of the wires which are 

 wound round the fields are not always connected up in the same 

 manner. In what is known as the series-wound Dynamo, the wire 

 is first connected with one of the brushes, woundround the fields, and 

 connected with one of the binding screws ; the other binding screw 

 is connected with the other brush. From these binding screws the 

 Electricity is led through wires to wherever it is required. This 

 kind of winding is the best adapted for lighting lamps arranged in 

 parallel arc where the resistance is not very great. 



In the shunt-wound as shewn in the diagram, the wire is first 

 connected with one brush, wound round the fields, and then instead 

 of being taken to the binding screw, connected to the other brush. 

 The Electricity is obtained in the main circuit by connecting each 

 of the brushes with one of the binding screws. This kind is best 

 adapted for lamps arranged in series, where there is a much 



