Chapter XXV: Part 2: (For core of 5.0 in. diam.)

2.00 120 2.22 160
3.00 158 2.93 215
4.00 194 3.58 265
5.00 228 4.22 310
8.00 324 6.00 450
10.00 384 7.10 530
12.00 450 8.30 625

The coil which you wound at the beginning of your experiment had only 75 turns and was tapped so that you could, by manipulating the two switches of Fig. 112, get small variations in inductance. In Table III is given the values of the inductance which is controlled by the switches of that figure, the corresponding number of turns, and the wave-length to which the antenna should then be tuned. I am giving this for two values of antenna capacity, as I have done before. By the aid of these three tables you should have small difficulty in taking care of matters of tuning for all wave-lengths below about 3000 meters. If you want to get longer waves than that you had better buy a few banked-wound coils. These are coils in which the turns are wound over each other but in such a way as to avoid in large part the "capacity effects" which usually accompany such winding. You can try winding them for yourself but I doubt if the experience has much value until you have gone farther in the study of the mathematical theory of radio than this series of letters will carry you.

TABLE III
Circuit of Fig. 112
Number Inductance in Wave length with antenna of
of turns. mil-henries. 0.0001 mf. 0.0002 mf.
14 0.04 120 170
20 0.07 160 220
28 0.12 210 290
36 0.18 250 360
44 0.25 300 420
56 0.38 370 520
75 0.60 460 650

In the secondary circuit there is only one capacity, that of the variable condenser. If it has a range of values from about 0.00005 mf. to 0.0005 mf. your coil of 60 turns and 0.42 mf. permits a range of wave-lengths from 270 to 860 m. Using half the coil the range is 150 to 480 m. With the larger condenser the ranges are respectively 270 to 1220 and 270 to 670. For longer wave-lengths load with inductance. Four times the inductance will tune to double these wave-lengths.

[Footnote 11: If you can afford to buy, or if you can borrow, ammeters and voltmeters of the proper range you should take the characteristic yourself.]