sentences:  The resulting inductor has a temperature coefficient of 6 to 8 parts per million per degree Celsius.Vackář points out that common air variable capacitors have a stability of 2 parts per thousand; to build a VFO with a stability of 50 parts per million requires that the variable capacitor is only 1/40 of the tuning capacity (.002/40 = 50ppm).The stability requirement also implies the variable capacitor may only tune a limited range of 1