High Frequency Apparatus Page 27

THE HIGH POTENTIAL TRANSFORMER watts lost through hysteresis. 27 Various Frequencies.-For practical purposes of the worker who builds the apparatus described in this book, the change in design necessary to adapt the various transformers for use on the various frequencies in commercial use, may be simplified so that when the design has been worked out for the 60 cycle instrument, the windings for 25 or 125 cycles become a simple matter of proportion. The cores may remain the same for all frequencies. Taking 60 cycles as the standard, the winding for 125 cycles may have just one-half the number of turns. The 25 cycle winding will have twice as many turns as the 60 cycle. In order to provide space for the additional turns of the 25 cycle winding, a wire one or two sizes smaller may have to be used but this is permissible in view of the intermittent work of the instrument. Proportions of Core.-The proportions of the core call for some plain common sense and rule-of-thumb calculation. One thing to bear in mind is that the core must not be made too long and slim as the reluctance is then too great and the primary and, consequently, the secondary will have an inordinate number of turns with relatively high copper cost. On the other hand, to make the core too short and thick renders the winding difficult of insulation and the coupling too close. Experience only can demonstrate the happy medium at first trial. The diagram given in Chapter IX shows a core of good proportions for this type of transformer and it may well be used as a pattern for instruments of larger or smaller size. Let us take the cross-section of the core at 2 inches square for a trial. We must have at least 94.33 cu. in. of iron in all. If we make the rectangle of the core 93/4 in. long and 63/4 in. wide, outside dimensions, we shall have 934 +94 +234 + 234 or 25 inches of core leg. The