单项选择题
The Wheat-stone bridge is a precision measuring instrument utilizing the principle of changes in（）.
A.inductance B.capacitance C.resistance D.amperage
单项选择题 Which of the motors for the devices listed below is fitted with an instantaneous overload relay?（）
单项选择题 One factor that determines the frequency of an alternator is the（）.
单项选择题 The device that most commonly utilizes the principle of electromagnetic induction is the（）.
