Wireless Telegraphy and Telephony

 56 WIRELESS TELEGRAPHY that the piston P is raised in the pump A it will draw in water through the small hole O. As soon as it descends, however, the water will reverse its direction and pass out. The action of the water represents that of an alternating current because it passes in first one direction and then in the other. The pump B is fitted with a valve whose action is to permit the water to flow in one direction only. The Fig. 69.—Pyron detector in which a fine wire is brought to bear against a crystal of iron pyrites. valve is fitted to the piston P'. It is a little flap which opens a hole in the piston when the latter is descending and closes when it is rising. Suppose that the piston is raised. Water will be drawn in through the little hole O'. As soon as the piston reaches the limit of the stroke it com- mences to descend. In falling it exerts a slight pressure on the valve which opens and allows the water to pass through. The hole in the piston is larger than the hole in the pump and so there is almost none of the water forced back into the pool. The next up stroke of the piston draws more water in, that which is on top flowing out through the overflow. The nature of the stream passing through