The auxiliary barrel _A_ is held to the anvil of the micrometer by means of a thumb screw _B_. At the inside end of the barrel is a secondary anvil _C_, the base of which bears against the short arm of the indicating lever _D_. The action will be clearly seen by reference to the engraving. The micrometer is so set that when a gage, _G_, of exact size, is placed between the measuring points, the long arm of the indicator stands at the 0 mark. If the pieces being calipered vary in the least from the standard size it will be readily noted by the movement of the pointer. Hard rubber shapes turned from rough casting often vary from 0.003 to 0.005 inch after having passed the inspector's test with an ordinary micrometer. With this attachment the inspector's helper can detect very minute variations from the limit size. Anything within the limits of the micrometer can be made to show to the naked eye variations as small as a ten-thousandth inch.[21] Another Sensitive Micrometer Attachment [Illustration: Fig. 43. Another Sensitive Micrometer Attachment] When testing the diameters of pieces that are handled in great quantities and are all supposed to be within certain close limits of a standard dimension, the ordinary micrometer presents the difficulty of having to be moved for each piece, and small variations in diameters have to be carefully read off from the graduations on the barrel. Not only does this take a comparatively long time, but it also easily happens that the differences from the standard diameter are not carefully noted, and pieces are liable to pass inspection that would not pass if a convenient arrangement for reading off the differences were at hand. Fig. 43 shows a regular Brown & Sharpe micrometer fitted with a sensitive arrangement for testing and inspecting the diameters of pieces which must be within certain close limits of variation. The addition to the ordinary micrometer is all at the anvil end of the instrument. The anvil itself is loose and consists of a plunger _B_, held in place by a small pin _A_. The pin has freedom to move in a slot in the micrometer body, as shown in the enlarged view in the cut. A spring _C_ holds the plunger _B_ up against the work to be measured, and a screw _D_ is provided for obtaining the proper tension in the spring. The screw and the spring are contained in an extension _E_ screwed and doweled to the body of the micrometer. A pointer or indicator is provided which is pivoted at _F_ and has one extensional arm resting against the pin _A_, which is pointed in order to secure a line contact. At the end of the indicator a small scale is graduated with the zero mark in the center, and as the indicator swings to one side or the other the variations in the size of the piece measured are easily determined. A small spring _G_ is provided for holding the pointer up against the pin _A_. The case _H_ simply serves the purpose of protecting the spring mentioned. As the plunger _B_ takes up