FIG. 83.—Dr. Seibt's rotary variable condenser. The plates are turned from a solid casting and the separation between is only .01 inch.
The advantage of an air dielectric is that no losses of energy take place through hysterisis. Hysterisis is the lagging which takes place in the process of charging and discharging. A thumb knob is fitted to the movable plates and provided with a pointer moving over a graduated scale so that the degree of capacity in use is indicated.
FIG. 84.—Sliding plate variable condenser.
In the sliding plate type of variable condenser the plates are either square or rectangular in shape and move back and forth in grooves cut in a hardwood frame as shown in the illustration.
Variable condensers are used for tuning and adjusting the receiving circuit in the same way that a tuning coil is employed, namely to increase or decrease the electrical length of the circuit so that it will respond to different wave lengths. The condensers are capable of finer adjustment than tuning coils because the change is gradual and even and is not in jumps from one step to another as from one turn to the next turn of the coil. If the desired point of resonance should happen to come between two wires of the coil and not in a position to be reached by the slider, the variable condenser can be adjusted to reach the exact degree of resonance and thus bring the circuit into finer adjustment than would otherwise be possible. The exact way in which this is accomplished and the effect upon the circuit will be left to the next chapter.
FIG. 85.—Diagram showing arrangement of rotary variable condenser in receiving circuit.
CHAPTER V. TUNING AND COUPLING, DIRECTIVE WAVE TELEGRAPHY.
Tuning has been mentioned in several places but not explained in any greater measure than was necessary to render a conception which would enable the reader to follow the text intelligently in order not to depart from the subjects under discussion there and consequently defeat the purpose of clearness.