Proceedings of the United States Naval Institute

rising and falling lines of force to generate a counter E. M. F. which will oppose it. If we cut off the direct current, however, the core will be no longer saturated and the alternating current will set up a reactance within the coil which will very effectively limit its value. The same principle is applied to the reactance key.

If sufficient direct current is passed through the primary of the key (transformer), the core becomes saturated and practically no reactance is offered to radio frequency current flowing through the secondary. When the primary is not excited, however, the secondary offers an extremely high reactance to radio currents. A double contact key similar to that shown in Fig. 10 is employed, so connected that when the key is up, contacts similar to B and C close the direct current circuit through the primary of the tank key and, A and B being separated, no current flows through the primary of the antenna key. This gives very low reactance to the tank circuit and very high reactance to the antenna with the result that the tank circuit oscillates and the antenna does not. With the key depressed, the reverse action takes place the tank has a high reactance and the antenna a low one. Thus the antenna is in effect connected to the arc and the tank removed.

While such a device is theoretically satisfactory, it happens in practice that due to residual magnetism, the transformers soon become sluggish in action and signaling is carried on only with difficulty. In addition, the actual resistance of the secondaries of the keys is so high that as great as 25 per cent of the radio current is wasted therein. While this is not harmful in the tank circuit where efficiency is not a desideratum, such a loss in the antenna current is a very serious matter and from the standpoint of efficiency cannot be permitted. The reactance key has had, therefore, a very limited adoption and has been discontinued in all recent installations.

This has necessitated a return to the compensation key but in high power installations, it has not been found practical to use the type previously described on account of the tremendous currents and potential to be broken. The inductive compensation key, K, shown in the dotted lines in Fig. 5 is the latest type of arc key to be evolved and has proved very successful. It consists, as shown, of a turn or two of wire inductively connected to the antenna loading inductance, arranged to be short-circuited by a relay key. When the key is depressed, these turns form a closed

circuit and hence exert an inductive influence on the antenna circuit due to the current induced in the former by that in the latter. The flux of this short-circuited secondary is counter to that of the primary or the antenna inductance, and hence reduces the latter's reactance and the radiated wave length. Its effect is thus seen to be similar to that of the conductive compensation key.

Since the potential in this closed circuit is too high to be broken at a single key, several keys in series are spaced evenly around the peripheries of the turns. The potential is divided uniformly between all of them and no undue flashing or arcing at the key contacts occurs. Since the impedance of this secondary is practically all inductive reactance-the resistance is made virtually nil by the use of very large, stranded wire-the current broken at the keys is practically wattless. Consequently, no greater sparking occurs at the key contacts of a 500 kw. arc transmitter than does at those of a 5 kw. spark set.

Arc sets have been built by the Federal Telegraph Company for naval installation ashore up to 500 kw. in size while for the radio station built by the navy at Croix d'Hins, France, a 1000 kw. converter was manufactured by the same firm. A photograph of the arc for the latter station is shown in Fig. 12. This type of arc was installed at the naval radio stations at Annapolis, San Diego, Pearl Harbor and Cavite and is the last word in high-power transmitters.

A description follows: AA-Magnetic core of field.

B-Exhaust pipe, connected to blowers for cleaning chamber.

C-Hose for conducting water supply to and from anode.

D-Anode support.

E-Anode insulating plate.

F-Lead to antenna inductance.

G-Tank containing field coils, insulated and cooled by circulating oil

supply.

H-Arc chamber.

I-Door to anode holder.

J-Pipe for hydrogen gas supply.

The cathode, not shown, is on the far side of the chamber.