Fig. 152. Details of Oscillation Helix Construction.
A hot wire ammeter placed in the aerial circuit of a transmitter employing an oscillation transformer will not indicate as much current as if placed in the same position in a circuit using an ordinary helix; but in spite of the fact, a transmitter using an oscillation helix will send farther because the energy is concentrated in waves of one length.
The construction of one type of oscillation helix has already been outlined on page 92. The form shown in Fig. 152 has no special advantages over the other but is preferred by many experimenters.
It is of the "pancake" type, so-called because of the flat form of the windings which are made in the shape of a spiral of brass ribbon set in a slotted frame.
The dimensions of the helix are clearly apparent from the drawing. The primary is composed of seven turns of brass ribbon 1/2 inch wide and 1/16 inch thick. The secondary should have from 10 to 15 turns of ribbon 3/8 x 1/16. The coils may be slid back and forth on the brass rod so that the distance between them is variable. Connection is made to the coils by means of suitable clips. A clip similar to that shown on page 92, but made to snap on a flat ribbon instead of a round wire, will serve the purpose.
QUENCHED SPARK GAP.
A "quenched" gap is made up of a number of brass or copper disks accurately turned to a true surface and separated by mica or rubber rings about .01 inch thick. The spark discharge takes place in the air-tight space at the center of the disks, inside of the mica rings.
The quenched gap has several advantages over other forms. It is practically noiseless and the nuisance of a crashing discharge may be avoided by its use.
The large surface offered to the spark by the disks cools the spark and quickly stops the oscillations in the closed circuit, and thereby leaves the open circuit and aerial system free to vibrate in its own period and therefore radiates pure waves. By pure wave a wave of one length is meant.
A quenched gap cannot be used on a set of over 1 K.W. power without artificial cooling by an air blast.