When the apparatus is in operation D C S is closed, so that the current from B operates the coherer (D C). Directly communication is broken off, the switch (D C S) should be opened; otherwise the buzzer would keep up a continuous tapping. For long-distance work a more efficient sending apparatus is shown in [Fig. 35]. This is composed of an induction-coil, with the terminal rods and brass balls forming the spark-gap, an oil key (K), and three or more large storage-cells, or a dynamo (if power can be had to run it). A condenser is placed in connection with the aerial and ground wires, so that added intensity or higher voltage is given the spark as it leaps across the gap. In operation this apparatus is similar to the one already described. Where contact is made with K the primary coil is charged, and by induction the current affects the secondary coil, the current or high voltage from which is stored in the condenser. When a sufficient quantity is accumulated the spark leaps across S G and affects wires A W and G W. This action is almost instantaneous, and directly the impulse sets the ether in motion the same impulse is recorded on the distant coherers and sounders.
There are a great many modifications of this apparatus, but the principles are practically the same, and while the construction of this apparatus is within the ability of the average boy, many of the more complicated forms of coherers and other parts would be beyond his knowledge and skill. Marconi has realized his ambition to send messages across the ocean without wires, and is now doing so on a commercial basis, and at the rate of twenty-five words a minute. It is but the next step to establish communication half-way around the world, and finally to girdle the earth.
Chapter X
DYNAMOS AND MOTORS
To adequately treat of dynamos and motors, a good-sized book rather than this single chapter would be necessary, and only a general survey of the subject is possible. Its importance is unquestionable; indeed, the whole science of applied electricity dates from the invention of the dynamo. Without mechanical production of electricity there could be no such thing as electric traction, heat, light, power, and electro-metallurgy, since the chemical generation of electricity is far too expensive for commercial use. Surely it is a part of ordinary education nowadays to have a clear and definite idea of the principles of electrical science, and in no department of human knowledge has there been more constant and rapid advance. It is only a truism to assert that the school-boy of to-day knows a hundredfold more about electricity and its varied phenomena than did the scientists and philosophers of old—Volta and Galvani and Benjamin Franklin. Yet it was for these forerunners to open and blaze the way for others to follow. A beginning must always be made, and the Marconis and Edisons of to-day are glad to acknowledge their indebtedness to the experimenters and inventors of the past. And now to our subject.
All dynamos are constructed on practically the same principle—a field of force rapidly and continuously cutting another field of force, and so generating electric current. The common practice in all dynamos and motors is to have the armature fields revolve within, or cut the forces of the main fields of the apparatus. There are many different kinds of dynamos generating as many varieties of current—currents with high voltage and low amperage; currents with low voltage and high amperage; currents direct for lighting, heating, and power; currents alternating, for high-tension power or transmission, electro-metallurgy, and other uses. It is not the intention in this chapter to review all of these forms, nor to explain the complicated and intricate systems of winding fields and armatures for special purposes. Consequently, only a few of the simpler forms of generators and motors will be here described, leaving the more complex problems for the consideration of the advanced student. For his use a list of practical text-books is appended in a foot-note.[3]
[3] First Principles of Electricity and Magnetism, by C. H. W. Biggs; The Dynamo: How Made and Used, by S. R. Bottone; Dynamo Electric Machinery, by Professor S. P. Thompson; Practical Dynamo Building for Amateurs, by Frederick Walker.
The Uni-direction Dynamo
The uni-direction current machine is about the simplest practicable dynamo that a boy can make. It may be operated by hand, or can be run by motive power. The field is a permanent magnet similar to a horseshoe magnet. This must be made by a blacksmith, but if a large parallel magnet can be purchased at a reasonable price so much the better, as time and trouble will be saved. This magnet should measure ten inches long and four inches and a half across, with a clear space seven inches long and one inch and three-quarters wide, inside measure. The metal should be half an inch thick and one inch and a quarter wide. A blacksmith will make and temper this magnet form; then, if there is a power-station near at hand where electricity is generated for traction or lighting purposes, one of the workmen will magnetize it for you at a small cost; or it can be wound with several coils of wire, one over the other, and a current run through it. When properly magnetized it should be powerful enough to raise ten pounds of iron. This may be tested by shutting off the current and trying its lifting power. If the magnet is too weak to attract the weight the current should be turned on and another test made a few minutes later.
Before the steel is tempered there should be four holes bored in the magnet and countersunk, so that screws may be passed through it and into the wooden base below, as shown at [Fig. 1]. This wooden base is fourteen inches long, eight inches wide, and one inch in thickness. It may be made of pine, white-wood, birch, or any good dry wood that may be at hand. The blocks on which the magnet rests are an inch and a quarter square and seven inches long. The magnet is mounted directly in the middle of the base, an equal distance from both edges and ends, as shown in the plan drawing ([Fig. 10]). The blocks are attached with glue and brass screws driven up from the underside of the base.