单项选择题
A direct current passing through a wire coiled around a soft iron core is the description of a simple（）.
A.magnetic shield
B.electromagnet
C.piezoelectric device
D.electromagnetic domain
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相关考题
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单项选择题
Which of the following electric motors would be the safest and most reliable to use on the main deck of a vessel in foul weather conditions?（）
A.Sealed motors
B.Drip proof motors
C.Enclosed motors
D.Watertight motors -
单项选择题
A resistance in a circuit of unknown value is to be tested using the voltmeter/ammeter methodTherefore, the meters should be connected with（）.
A.both meters in series with the resistance
B.both meters in parallel with the resistance
C.the ammeter in series and the voltmeter in parallel with the resistance
D.the ammeter in parallel and the voltmeter in series with the resistance -
单项选择题
An accidental path of low resistance, allowing passage of abnormal amount of current is known as a/an（）.
A.open circuit
B.short circuit
C.polarized ground
D.ground reference point
