Inductance in Number Length Feet of wire mil-henries. of turns. in inches. required. 0.10 25 0.46 25 0.16 34 0.63 36 0.20 39 0.72 42 0.25 44 0.81 49 0.40 58 1.07 63 0.60 75 1.38 80 0.80 92 1.70 100 0.85 96 1.78 104 1.00 108 2.00 118 1.20 123 2.28 133 1.80 164 3.03 176 2.00 180 3.33 190 3.00 242 4.48 250 4.00 304 5.62 310 5.00 366 6.77 370 8.00 550 10.20 550 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.