Page:Popular Science Monthly Volume 91.djvu/331

This page needs to be proofread.

Popular Science Monthly



��jj ] [Jtransf



��America's New Semi- Wireless System for Telephoning from Captive Balloons

WHAT will be still another addition to the long list of America's contribu- tions to the technique of modern warfare is a Mtt'raZ-wireless system for communi- cating from balloons. The gun-fire of artillery is directed, as everybody knows, by battery commanders who ascend to high altitudes in anchored balloons. From here the commanders take their observa- tions and telephone their orders down to their batteries which may be concealed several miles away. To provide suitable telephone transmission lines, the Allies have been using the wire cables which anchor the balloons. They have built these cables up with an internal steel core which they insulate from the outer strands. But in providing two insulated conductors in this way, they were com- pelled to use a bulky, very expensive cable which was likely to be rendered, worse than useless if too roughly handled. For if once this cable should become injured and the two conductors become short-cir- cuited, the telephones could not work and the commander would be cut off from his men.

The semi-wireless system which has just been brought out by William Dubi- lier and Robert Goll, two American engineers, does away with any need for the internal core. Their system, which is fully protected by patents, is expected to be officially adopted by our Govern- ment. For connecting paths between the balloon and the ground, the new system uses an ordinary solid steel cable as one path, and the air itself as the


��other. It therefore uses one transmission line, as in wire telephony, and it also uses the air as in wireless.

Considering the diagram of con- nections it will be seen that the system has one secondary circuit consisting of the secondaries of the two telephone transformers, of the two telephone receivers and their shunt condensers, and of the two metal sheets which, with the air be- tween as their dielectric, form a balanc- ing capacity. This one secondary has two primaries : the telephone transmitter, batteries and the primary of the trans- former, in the basket, and the similar instruments down below at the gun battery. The three circuits are mutually tuned to provide a maximum of current in the secondary.

When the commander talks into his transmitter, the corresponding primary current is modulated accordingly. The variations in the primary currents are induced into the secondary through the step-up transformer. Here the variations become very marked, due to the strength- ening effect afforded by the' inductance of the transformers and the capacity of the air-condenser, which together natur- ally tend to cause the secondary current to oscillate. These variations are changed into sound waves by the telephone re- ceivers — and the gun captain thus re- ceives his orders.

��A Polarity-Changer for Reversing Lighting Battery Current

SOME audion detectors work better if the current from the lighting battery is reversed. To do this quickly a polarity- changer is very bandy. There are many kinds of polarity-changers, but the one here described is very compact and looks neat. To make it, mount five contact points on a fiber-base as shown in the drawing on the following page; space them about half their diameter apart. The positive side of the battery is connected with No. I and 5; the negative with No. 3, while No. 2 an^l 4 are connected with the fila-


�� �