The usual construction is to have a stationary field-magnet and then a series of electromagnets mounted and revolving upon a shaft in the center of the magnetic field. The rotating part is called the armature, and is so wound with insulated wire that successive induced currents are created in the armature windings and discharged through brushes which rest on revolving segments that connect with the armature windings. These induced currents succeed each other with such rapidity as to amount in practice to a steady current. However, the separate pulsations are easily heard in any telephone when the circuit is near to that of a dynamo circuit. The dynamo current is not nearly so steady as the battery current, although both are probably made up of separate discharges. In the dynamo there is a discharge every time the electromagnet of the armature cuts through the lines of force of the magnetic field, and in the galvanic battery every time a molecule is broken up and its little measure of energy is set free. In the dynamo the pulsations are so far apart as to make a musical tone of not very high pitch, but in the galvanic battery the pitch of the tone, if there is one, would require a special ear to hear it--one tuned, it may be, up near the rate of light vibration. There are two types of dynamo, one generating a direct and the other an alternating current. (By alternating we mean first a positive and then a negative current impulse.) We cannot enter into a technical description of the dynamo in a popular treatise such as this. The dynamo has evolved from the germ discovered by Faraday, till to-day it is a machine, the construction of which requires the highest class of engineering skill. When in action it seems like a great living presence, scattering its energy in every direction in a way that is at once a marvel and a blessing to mankind. But we must not give all the credit to the dynamo. As the moon shines with a reflected light, so the dynamo gives off energy by a power delegated to it by the steam-engine that rotates it, and the steam-engine owes its life to the burning coal, and the burning coal is only giving up an energy that was stored ages ago by the magic of the sunbeam; and the sun--? Well, we are getting close on to the borders of theology, and being only scientists we had better stop with the sun.