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43
Radio Broadcast
The formula from which Chart i was constructed is
X= 18847EC
where X represents the wavelength in meters, L the inductance in microhenries, and C the capacity of the tuning condenser in microfarads.
Referring to Fig. 1, it is seen that the two charts have been placed side by side, with the L-scale in common. As an example of the use of these charts, let us say that we wish our tuned circuit to reach a maximum wavelength of 550 meters, and that we plan to use a .0005 mfd. variable condenser tubing three inches in diameter, and No. 18 d. c. c. wire. With a pencil and ruler we draw a line from .0005 on the C-scale through 550 on the X-scale, until it intersects the L-scale, reading 170 mh., as the required inductance of our coil. Another line is drawn from L = 170 to d = 3.
Referring to the copper wire table, Fig. 2, we find that No. 18 d. c. c. wire can be wound 19.6 turns to the inch, so we draw a third line from this value on the n-scale, through the intersection of the second line and the index line, until it intersects the l-scale. This shows us that the coil should be wound to a length of three inches, and the product of n and 1 gives 59 as the number of turns required. I
The chart may be worked in the reverse direction in exactly the same manner, always making sure that the correct pairs of scales are connected together.
The amateur builder will find many, uses for these charts. In addition to their value in designing inductances for a new receiver, they may be used to redesign a coil for different sizes of tubing, wire, or tuning condenser than originally specified.
KIND OF INSULATION
B. & S. GAUGE
ENAMEL
DCC
sec
DSC
ssc
ENAMEL
AND
AND
sec
ssc
14
13
7
14.6
14.7
15
0
15.2
14.2
14.7
15
15
0
16.2
16.4
17
0
17.0
15.8
16.5
16
16
7
18.0
18.2
19
0
18.7
17.6
18.4
17
18
5
20.0
20.0
21
.2
21.4
19.5
20.5
18
19
6
22.3
22.3
23
6
24.0
21.7
22.9
19
22
5
25.0
25.2
27
0
27.2
24.2
25.8
20
24
5
27.5
27.5
29
5
30.1
26.5
28.4
21
27
5
30.8
30.8
32
8
33.6
29.6
31.5
22
30
0
34.0
34.0
36
6
37.7
32.7
35.0
23
32
7
37.5
37.5
40
7
42.3
36.1
39.0
24
35
5
41.5
41.5
45
3
47.2
39.7
43.1
25
38
5
45.7
45.7
50
3
52.9
43.7
47.9
26
41
8
50.2
50.2
55
7
59.0
47.8
52.8
27
45
0
55.0
55.0
61
7
65.8
52.1
58.1
28
48
5
60.0
60.0
68
3
73.9
57.0
64.4
29
52
0
65.5
65.5
75
4
82.2
61.9
70.6
30
55
5
71.3
71.3
83
1
92.3
67.4
77.9
31
60
0
77.3
77.3
91
6
103.0
72.8
85.3
32
62
7
83.7
83.7
101
0
116.0
79.1
93.9
33
66
3
90.3
90.3
110
0
130.0
85.6
103.0
34
70
0
97.0
97.0
120
0
145.0
91.7
112.0
35
73
4
104.0
104.0
131
0
164.0
98.8
123.0
36
77
0
111.0
111.0
143
0
182.0
105.0
133.0
37
80
3
126.0
126.0
155
0
206.0
113.0
146.0
38
83
5
133.0
133.0
168
0
235.0
120.0
157.0
39
89
7
140.0
140.0
181
0
261.0
128.0
172.0
FIG. 2 Wire turns per linear inch