Dial indicators may be used to check the variation in tolerance during the inspection process of a machined part, measure the deflection of a beam or ring under laboratory conditions, as well as many other situations where a small measurement needs to be registered or indicated. The definition of small depends on the observer however a range between 1 mm (0.040") and 50 mm (2") may be thought of as typical with a travel of 10 mm (approx 0.5") being perhaps the most common.
Probe indicators typically consist of a graduated dial and needle (thus the clock terminology) to record the minor increments, with a smaller embedded clock face and needle to record the number of needle rotations on the main dial. The tool may be graduated to record measurements between 0.01 mm (.001", which is not a direct unit conversion) down to 0.001 mm (.00005") for more accurate usage. The probe (or plunger) moves perpendicular to the object being tested by either retracting or extending from the indicator's body.
The dial face can be rotated to any position, this is used to orient the face towards the user as well as set the zero point, there will also be some means of incorporating limit indicators (the two metallic tabs visible in the right image, at 90 and 10 respectively), these limit tabs may be rotated around the dial face to any required position. There may also be a lever arm available that will allow the indicator's probe to be retracted easily.
A Dial Test Indicator, also known as a Lever arm test indicator, or Finger indicator has a smaller measuring range than a dial indicator and therefore has the ability to measure in smaller increments. A test indicator measures the deflection of the arm, the probe does not retract but swings in an arc around its hinge point. The lever may be interchanged for length or ball diameter and permits measurements to be taken in narrow grooves and small bores where the body of a probe type may not reach. The model shown is bidirectional, some types may have to be switched via a side lever to be able to measure in the opposite direction.
These indicators actually measure angular displacement and not linear displacement. If a force is perpendicular to the finger, the linear displacement error is acceptably small within the display range of the dial. However, this error starts to become noticeable when the force is as much as 10 degrees off the ideal 90. These indicators are used to compare two surfaces and alert the user when they are at the same position relative to the body of the indicator. In this application the force can be applied at almost any angle since you are looking for the same reading on two different surfaces.
Some cheaper models of test indicators may come with a pear shaped probe in an attempt to compensate for cosine error and a multiplier or correction factor will not have to be used. Contact points of higher end test indicators such as Mitutoyo, Brown & Sharpe, TesaTast and Interapid come with a standard carbide sphere at 1, 2 or 3mm diameter. More usable materials are available for contact points such as ruby, teflon and PVC. These are more expensive and not available OEM, but, are extremely useful in applications where surfaces are easily scratched or damaged.