Fig. 28 shows a form of surface gage that has proved very handy, and which can be used also as a height gage for measuring distances from shoulders to the base. If accurately made it is equal, and often preferable, to the vernier or slide caliper now so generally used with an attachment to the sliding jaw. One of its advantages over the vernier is the readiness with which the graduations are discerned, and it is as easy to manipulate as the ordinary micrometer. The part _B_, which forms the main body of the instrument, is made of tool steel, and one end is fitted into the base where it is held in position by the screw _D_. The remainder is milled to a thickness of 1/8 inch and has graduations of 0.025 inch for a distance of three inches. The screw _A_ is the most essential part of the tool, and its construction requires great accuracy. Its diameter is ½ inch, and it is cut with 20 threads per inch. In the upper end of the screw is driven the ball _H_ for the sake of giving a neat appearance. The top of the thread is turned off 0.010 inch to allow the scriber _F_ to slide freely on the screw. The barrel _I_ is used for raising and lowering the slide, but instead of having the graduations placed directly upon it, they are made upon the sleeve _C_, which fits over a shoulder on the barrel. This allows more easy means of adjustment than would be possible were the graduations placed on the barrel itself. The sleeve is graduated with fifty divisions each equaling a movement of the scriber of 0.001 inch. This sleeve may be turned by means of a small spanner wrench so as to bring the zero line into correct position to compensate for wear. A knurled locking nut is also provided for holding the scriber in any fixed position. The scriber itself is hardened and lapped to a finished surface, the tail end being slotted and provided with two screws to compensate for wear. On the scriber is placed the zero mark which shows at a glance the measurement that is being taken. The block _K_ is three inches in height, and by using this block and placing the gage on its top, the range of the gage is increased to six inches. The screw _E_ is used for fastening the gage to the top of the block. The center of the block is drilled out and slots cut through the sides in order to make it light and neat in appearance.[15] Micrometer of from One- to Five-inch Capacity [Illustration: Fig. 29. Micrometer of from One- to Five-inch Capacity]