We have studied how electricity is generated by chemical means in batteries and by friction. These two forms of electrical energy are very valuable for commercial purposes where a small current is sufficient, such as is necessary for ringing electric bells, etc. The current generated by these two methods is not, however, strong enough to drive large machines or to light lamps. The commercial method of generating electricity on a large scale is by means of a machine called a dynamo or generator. The principal parts of a dynamo are: (1) the magnetic field, produced by permanent magnets or electromagnets; and (2) the armature, which consists of a moving coil or coils of wire wound on a revolving iron ring or drum.