| 摘要 |
The apparatus comprises a fixed film-holder (1), a lens-holder (2) and a document-holder (3), the lens-holder being adjusted manually into the position corresponding to the required ratio n, while the document-holder is locked (slaved) to the control-circuit mounting. The control circuit comprises a Wheatstone bridge (8) and an amplifier (15) with two inputs connected respectively to the first and second opposite corners (13, 14) of the Wheatstone bridge and with an output connected, via a reversing switch (17), to the correction motor (5). The third and fourth opposite corners (18, 19) of the bridge are connected respectively to the ends of a potential divider bridge comprising two resistors (21, 22) in series, each having the value F OMEGA . The junction point of these two resistors is connected to an input of a comparator (23) whose other input is connected to one (13) of the first and second opposite corners of the bridge, the output of this comparator being connected to the reversing switch in order to reverse the polarity of the error signal applied to the motor by the amplifier when the ratio n passes from a value greater than 1 to a value less than 1, or vice versa. <IMAGE>
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