Methods: The thickness of enamel and dentin at different positions of 23 upper first and 18 lower first molars were measured by electronic ruler.
方法：用电子测量尺测量23颗上颌第一磨牙和18颗下颌第一磨牙牙合面不同部位牙釉质、牙本质的厚度。
Also it analysed the cause of the faults of the electronic displaying ruler in sound speed measuring unit.
对于声速测定仪中电子数显尺的常见故障原因进行了分析，并总结出维修方法。
Also it analysed the cause of the faults of the electronic displaying ruler in sound speed measuring unit.
对于声速测定仪中电子数显尺的常见故障原因进行了分析，并总结出维修方法。
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