This arrangement makes it possible to secure any voltage from one-half to ten in one-half-volt steps from the secondary of the machine. Each step on the switch, _D_, will give two volts, while those on _E_ will each give one-half of a volt. [Illustration: Fig. 189.—The Complete Switch.] Two binding-posts (marked _P_ and _P_ in the drawing) mounted in the upper corners of the base are connected to the terminals of the primary winding. The two posts in the lower corners (marked _S_ and _S_ in the drawing) are connected to the switch levers, and are the posts from which the secondary or low voltage is obtained. [Illustration: Fig. 190.—Diagram of Connections.] The transformer may be connected to the 110-V. alternating current circuit by means of an attachment plug and cord. One end of the cord is placed in each of the primary binding-posts. The other end of the cord is connected to the attachment plug so that the latter may be screwed into any convenient electric-light socket. [Illustration: Fig. 191.—Top View of the Transformer.] The transformer must not be connected directly to the line. An instrument such as this is not designed for continuous service and is intended to be disconnected as soon as you are through using it. [Illustration: Fig. 192.—Side View of the Transformer.] It will be found a great convenience in operating many of the electrical devices described, wherever direct current is not essential. CHAPTER VIV WIRELESS TELEGRAPHY Probably no branch of electrical science ever appealed more to the imagination of the experimenter than that coming under the heading of wireless telegraphy. Wherever you go, you are likely to see the ear-marks of _amateur_ wireless telegraph stations in the aerials and masts set up in trees and on house-tops. It is estimated that there are nearly a quarter of a million such stations in the United States. There is really no great mystery about this wonderful art which made possible the instantaneous transmission of messages over immense distances without any apparent physical connection save that of the earth, air, or water. Did you ever throw a stone in a pool of water? As soon as the stone struck, little waves spread out from the spot in gradually enlarging circles until they reached the shore or died away. By throwing several stones in succession, with varying intervals of time between them, it would be possible so to arrange a set of signals, that they would convey a meaning to a second person standing on the opposite shore of the pool. Wireless telegraphy is based upon the principle of _creating and detecting_ waves in a great _pool_ of ether. Modern scientists suppose that all space is filled with an "imaginary" substance called _ether_. The ether is invisible, odorless, and practically weightless. This ether, however, bears no relation to the anaesthetic of that name which is used in surgical operations. It surrounds and penetrates all substances and all space.