The dynamo is the form of generator now in general use where heavy currents of electricity are needed. It is aptly described by a writer in Modern Machinery, Mr. John A. Grier, as a thing that when "at rest is a lifeless piece of mechanism; in action it has a living spirit as full of mystery as the soul of man." This is a poetic way of describing it that conveys to the mind a sense of the power and beauty of natural law in action, that would not come from a mere recital of the cold scientific facts. The facts, however, are necessary: but let us draw from them all the poetry and all the practical lessons that we can as we go along; for it is this blending of the poetic with the practical that lends a charm to our every-day "grind," and lightens the load of many a weary hour. The dynamo is a machine that converts mechanical into electrical energy, and the great practical value of energy in this form is that it can be distributed through a conductor economically for many miles. We can transmit mechanical power by means of a rope or cable for a limited distance, but at tremendous loss through friction. We can transmit power through pipes by compressed air or steam, but there is a great loss, especially in the case of steam, by condensation from cold. None of these methods are available for long distances. Another advantage electricity has over other forms of energy is the speed with which it can be transmitted from one place to another. In this respect it has no rival except light. But we have not been able to harness light and make it available to carry either freight or news, except in the latter case for a short distance by flashing it in agreed signals.