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现代化的电子通讯是一个成员众多的大家族,其中资格最老、数量最多的要算有线电话了。现在,有的国家平均每100人就有50～60部电话。个别工业发达的国家电话已普及到每个家庭,甚至人手一部。当今的电话,种类繁多,除常见的拨号电话外,又出现了新式的多功能电话。如既能闻声,又能自动抄写、记录的小型书写电话;数字保密电话;能自动录音的录音电话;既有声音又有图象的电视电话,等等。高级指挥员有了电视电话,就跟身临战场一样,可以直接观察和指挥现代化的战争。但是,有线电话也有弊病:它容易被人窃听。这是为什么呢?原来,在打有线电话的时候,人的声音信号要通过电话机变成话音电流,再送到电话线路上去,它以每秒30万公里的速度传播。这时,在金属导线周围,就产生一个磁场,其强度随着话音电流的大小而起伏变化,只要线路中有话音电流通过,它就成为一个电磁波辐射源,向线路附近的空间
Modern electronic communications is a large family of members, of which the oldest, the largest number to count the landline. Nowadays, some countries have 50 to 60 phones for every 100 people on average. Some industrialized developed country phones have spread to every family or even a manpower section. There are a wide variety of phones today, with the addition of new multifunction phones in addition to the usual dial-up calls. If you can hear the sound, but also automatic transcribe, record the small writing telephone; digital security telephone; automatic recording voice telephone; both voice and image of the TV telephone, and so on. With the help of a videocall, senior commanders, just like the battlefield, can observe and direct the modern war directly. However, there are disadvantages with wired phones: it can be easily eavesdropped. Why? The original, when making a landline call, the voice signal of a person is turned into a voice current through the telephone and then sent to the telephone line, which propagates at a speed of 300,000 kilometers per second. At this moment, a magnetic field is generated around the metal wire and its intensity fluctuates with the magnitude of the voice current. As long as the voice current passes through the wire, it becomes an electromagnetic wave radiation source, and the space around the circuit