FIGURE

If, now, the center point of a humped cardboard template be laid on the chart at the point where a vertical line corresponding to the frequency indicated as above intersects a horizontal line representing the Qm characteristic of this structure independent of air gap (passing through the peak of the original humped curve), one can read the Q of coils on this structure (having the chosen air gap) at any other frequency.

To derive the third curve from the second, use is made of the relationship that Qm occurs at a frequency which is inverse with effective permeability, i.e., with inductance. The product, then, of inductance and frequency for Qm at all points is the same. The value of this

Figure 2. A coil wound on a small,or postage-stamp, core {GR 746) of 29-gauge 4% silicon-steel laminations with an wide center leg stacked high was measured at 1 kc, but the plot was quite lopsided, with most of the points on the low-inductance side of the peak. The frequency should have been higher.

Figure 3. A coil on the same core as that of Figure 2, employing 29-gauge A-metal laminations, was measured at 1 kc, which was nearly the correct frequency.

Figure 4. A coil on a 345 core of 26-gauge 4% silicon-steel laminations, having a square center leg, was measured at ZOO cycles. The laminations had a blunt-angled nose. (See Figure 12.)

Figure 5. The same coil as in Figure 4, but with square-nosed laminations. Unfortunately, they were available only with a center-leg air gap no smaller than Jjg". Therefore, there are no points between approximately 0.25 henries (corresponding to product can be obtained by multiplying the frequency of measurement by the inductance at which maximum Q of the humped curve occurs. If Figure 8 is taken as an example, the maximum of the humped curve occurs at 0.245 henries, and the measuring frequency is 400 cycles. The product of the two is 98. The air gap corresponding to 0.245 henries is 68 mils at 400 cycles. To obtain a point on the third curve corresponding to any point on the second curve, divide the abscissa for the second curve into 98, from which will be obtained the abscissa for the third curve. The ordinate is the same for both. The end result is two curves, mirror images of one another.

air gap) and 1.3 henries (corresponding to completely interleaved laminations). The two sloping curves are short for the same reason. Within the range where both have points, the curves of Figures 4 and 5 should be and actually are sensibly in the same locations:

Figure 6. A coil on the same size core, but made from 29-gauge A-metal laminations, was measured at 400 cycles.

Figure 7. A coil on a 485 core of 26-gauge 4% silicon-steel laminations, having a square center leg, was measured at 400 cycles.

Figure 8. A coil on the same size core, but made from 29-gauge A-metal laminations, was measured at 400 cycles.

Figure 9. A coil of 8800 turns of No. 30 enamel on 365 core of 26-gauge 4% silicon-steel laminations, having a ljfg" square center leg, was measured at 400 cycles.

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