Page:Popular Science Monthly Volume 91.djvu/172

This page needs to be proofread.


156

��Popular Science Monthly

��A Trussed Aerial Spreader for Long Wires

ALIGHT and strong spreader is very desirable when the aerial reaches over 200 ft. in length. While bamboo answers the purpose for spreaders shorter than 6 ft., it does not do for longer ones.

��WIRE

���A light frame trussed so that it will be strong enough to hold a large aerial

A truss-built spreader fills the conditions admirably. Select two straight-grained pieces of spruce 9 ft. 3 in. long, by 1 in. square and two braces 6 in. long by 1 in. square. The pieces are assembled as shown in Fig. 1. The detail showing how the braces are fastened to the spreader- piece on the side where the aerial wires are attached is seen in Fig. 2. The other ends of the braces are fastened with wood screws. The rope-bridle is fastened 18 in. from each end, to equalize the strain, as shown in Fig. 3. The arrows denote the line of strain. A spreader built with these dimensions is sufficiently large for an aerial 300 ft. in length.— E. R. Thomas.

��Position of Wireless Waves Passing Over Land

WHEN radio waves travel along the surface of the sea, or of any other good conductor, their fronts stand up nearly vertically. When they pass across stretches of poorly-conducting earth, how- ever, the tops tend to gain and the whole wave-front tips forward in the direction of motion. Resulting currents in the surface of the earth cause resistance losses, and the waves rapidly become weaker. This is why it is more difficult to send wireless signals over ground than over salt water.

��Cloudy Days Best for Wireless Wave Signals

MEASUREMENTS made at the Uni- versity of North Dakota showed that on the night following a cloudy day signals were received much more clearly than on nights following days of bright sunlight. It appeared that the cloudiness was most effective when it covered the territory lying between the sending and receiving stations.

��Improving the Tone of a Test Buzzer Used on Wireless Detector

THE tone of a buzzer used in finding a sensitive spot on the crystal detector can be made high-pitched by inserting a piece of paper, folded four times, between the contact spring and the bar next to the magnet. Also insert a folded piece between the cone of the first coil and the bar.

��Simple Construction of a Rotary-Gap Disk

IN the accompanying illustration is shown a new type of rotary-gap disk which will give unusually good results. It is very easy to construct. First procure a piece of 3^-in. sheet fiber and cut out a disk 9 in. in diameter. With a 4-in. radius draw a circle on this and divide it off into 8 equal

���Holes in a fiber disk to allow the spark to jump between the electrodes as it turns

parts. Drill holes on these marks slightly larger than the gap-electrodes and drill a hole in the center for the shaft. Mount the disk on a motor in the usual way with a set- screw or clamp, and mount a stationary gap as shown at A. Every time a hole is passed, the gap is permitted to spark. More holes can be added if desired, depending on the speed of the motor.

�� �