| 摘要 |
PURPOSE:To hold the heating value of a focus coil invariably constant even when ranges are changed by providing plural focus coils and connecting the focus coils by a changeover switch so that mutual connection directions are changeable. CONSTITUTION:When the numbers of turns of the focus coils 2a and 2b are denoted as N1 and N2, the changeover switch 13 is operated as shown by a solid line and then they are connected to a current direction switch 8 in such a direction the numbers of turns of the focus coils 2a and 2b added together, thereby obtaining the number N1+N2 of turns on the whole of the focus coils. When the changeover switch 13 is operated as shown by an alternate long and two short dashes line, the number N1-N2 of turns of obtained on the whole. Therefore, measurement ranges are changed through the operation of the changeover switch 8 without varying the level of a supplied current and even when the ranges are changed, the heating value of the focus coils is held invariably constant.
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