| 摘要 |
<p>PCT No. PCT/CH96/00017 Sec. 371 Date Dec. 3, 1997 Sec. 102(e) Date Dec. 3, 1997 PCT Filed Jan. 15, 1996 PCT Pub. No. WO96/25000 PCT Pub. Date Aug. 15, 1996A telephone station (1) having a handset (14) can be connected to a telephone exchange (5) by means of first and second switches (6, 7) by way of two lines (2, 3). When the switches (6, 7) are closed the telephone station (1) can be supplied with electrical energy from the telephone exchange (5) by way of the lines (2, 3). Within the telephone station (1) the first switch (6), a first coil (8), a circuit member (10), a second coil (9) and the second switch (7) are electrically connected in series. The switches (6, 7) are actuable by the handset (14) of the telephone station (1). The two coils (8, 9) are wound in opposite directions around a body (12) of magnetic material. There are also provided a measuring coil (11) and a measuring circuit (13) for producing an alternating magnetic field in the body (12) and for measuring the time derivation dB/dt of the magnetic induction B(t) which occurs in the body (12) and for producing a signal for opening the switches (6, 7) if the measured derivation dB/dt fulfills predetermined criteria. The coils (8, 9), the magnetic body (12), the measuring coil (11) and the measuring circuit (13) serve to prevent attempts at fraud.</p> |
