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