High Frequency Apparatus Page 31

THE HIGH POTENTIAL TRANSFORMER 31 Likea tap at the 231st turn for the 70-volt connection. wise, for 220 volts, the number of turns would have to be doubled and in this case the wire would need to have but half the area. This would be No. 17 wire which has an area of 2,048 circular mils. For a maximum of convenience and adaptability with a minimum of complication, the winding may be of 363 turns of No. 14 wire, tapped at 231 turns, and then upon the No. 14 wire may be placed an additional winding of 363 turns of No. 17 wire with its starting end joined to the finishing end of the first winding. This primary permits the transformer to be used on 70, 110, and 220 volts without any change other than a simple connection. The secondary turns we know to be 16,480. No. 32 enameled wire is suitable for this winding and this wire winds 112 turns per inch. Suppose we make each layer of secondary wire contain 230 turns; this will bring the width just over 2 inches which allows a good space for insulation from the core. Seventy-two layers of wire will give us 16,560 turns which is near enough to the stipulated 16,480. Perhaps for the sake of having finishing and starting turns come on opposite sides of the winding, it may be well to wind but 71 layers which will give 16,230 turns. This procedure is allowable and, indeed, preferable, as the difference of a hundred-odd turns in the secondary will have no appreciable effect upon results. The calculations for the weight of primary and secondary wire are obvious. If the coils are wound upon round forms, as they may well be, the average length of turn is easily determined and multiplied by the total number of turns. This reduced from inches to feet gives, on comparison with the wire tables, the weight of the wire in pounds. Induction Coils.-In places where the 110-volt lighting current is not available, a battery of generous proportions