單項(xiàng)選擇題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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1.單項(xiàng)選擇題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

2.單項(xiàng)選擇題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

3.單項(xiàng)選擇題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

4.單項(xiàng)選擇題Which of the following statements represents the main difference between a relay and a contactor?（）

A.Contactors control current and relays control voltage
B.A relay is series connected and a contactor is parallel connected
C.Contactors can handle heavier loads than relays
D.Contactors are made from silver and relays are made from copper

5.單項(xiàng)選擇題Any electric motor can be constructed to be（）.

A.short proof
B.ground proof
C.explosion proof
D.overload proof