2. 自考
  3. The electromagnet was invented in England by William Sturgeon，who took an iron rod and bent it into the shape of a horseshoe．Thi

The electromagnet was invented in England by William Sturgeon，who took an iron rod and bent it into the shape of a horseshoe．Thi

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问题     The electromagnet was invented in England by William Sturgeon，who took an iron rod and bent it into the shape of a horseshoe．This“horseshoe”was coated with varnish and a layer of copper wire was wrapped around it．An electric current was passed through the wire，thus making the rod magnetic．The rod was now，because of magnetic attraction，able to support nine pounds of iron．In the US，a scientist named Joseph Henry improved on Sturgeon’s electromagnet by insulating the copper wine with silk．He was able to wrap many turns of wire around an iron core without danger of short circuits between the turns．His magnet could hold 2,300 pounds．This experiment prompted Henry to try his hand at converting magnetism into electricity．First he coiled some insulated wire around an iron bar．connecting both ends of the wire to a galvanometer(电流表)．The iron bar was placed across the poles of the electromagnet．Then the coil of the electromagnet was connected to a battery．The galvanometer indicated a voltage，then dropped to zero．Henry signaled his assistant to disconnect the coil．The galvanometer showed that once again a voltage had been produced，although this time in the opposite direction．The principle of electromagnetic induction had thus been discovered．Unfortunately for Joseph Henry he did not publish his findings and someone else(Faraday)got the credit for the discovery．   
In Henry’s experiment，the galvanometer indicated a voltage when______．
选项 A、the coil was connected to a battery
B、the coil was disconnected to a battery
C、neither A nor B
D、both A and B
答案D
解析 答案见文章10～15句。
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