FIG. 88.—Hot-wire ammeter.
Helix and tuning coils are divided into the "inductive" or "loose" and the "direct" or close coupled types. Inductive tuning coils are known as "loose-couplers" and "receiving transformers." Inductive helixes consist simply of two helixes, separated from one another as shown in the accompanying illustration. The upper helix, called the secondary, can be raised or lowered upon a central support. Varying the distance between the primary and secondary is varying the "coupling." There are several advantages derived by using loose coupled sending helixes, the chief of which lie in the fact that it is possible to radiate larger amounts of energy and also decrease the "damping."
FIG. 89.—The principle of the hot-wire ammeter.
In order to tune a transmitter, the "hot-wire" ammeter is necessary. This instrument makes use of the property which electrical conductors possess to become heated and expand when a current is passed through them.
The accompanying diagram serves to illustrate the principle of the "hot-wire" meter. A piece of platinum wire is stretched tightly between two rigidly fixed posts. A thread leads from the center of the "hot wire" to a small spindle around which it passes once or twice. The spindle is also connected to a spring which exerts a continual tendency to turn the spindle but is prevented from so doing by the thread attaching to the hot wire. Any tendency on the part of the string to slacken a little, however, will immediately permit the spring to turn the spindle. When a high frequency current is passed through the platinum wire it becomes heated and expands. The expansion of the wire allows the thread to slacken slightly with the immediate result that the spindle turns. The spindle carries a pointer at the upper end which shows the amount of turning. It is therefore easy to tell the comparative strength of current flowing accordingly as the deflection is great or small.
FIG. 90.—Diagram showing loose coupled helix in circuit.
The meter is placed in series with the aerial and when the high frequency currents pass through it they heat and expand a fine wire, causing the needle to move over a graduated scale and indicate the amount of current passing. The apparatus is "tuned" or in resonance when the length of the spark gap, the condenser and the helix have been so adjusted that the oscillations flow freely through the system and the maximum amount of current is indicated by the ammeter.
