TABLE OF CONTENTS Indicators are displacement sensing, amplifying and indicating or displaying instruments. Their purpose is (a) to detect variations in a specific distance, as determined by a reference plane established at a fixed position relative to the instrument and by a selected gaging point on the surface of the object, and (b) to display on a dial, graduated scale or digital display the amplified version of the sensed dimensional variations.
When the measurement of length is the objective of the process, the indicator whether mechanical or electronic will typically be used to compare the actual length of the specimen to the length of another physical body. As such, indicators are typically thought of and used in connection with the process of comparative gaging. However, recent advances in electronic indicators, as discussed later in this chapter, make absolute length measurements possible. Yet, when an indicator is used in comparative measurement, a master of known size is substituted for the nominal length of the dimension to be gaged. Thus, the indicator instrument only measures the amount and shows the deviation that may exist in relation to the nominal size.
When devising or evaluating indicator measurements of length, it must be remembered that the indicator in itself is not a complete measuring instrument. As shown diagrammatically in Fig. 5-1, for the purpose of length measurements the indicator must be complemented with devices serving the following functions:
Staging the object on a reference plane...
