For example: Suppose the diameter of the jaws to measure, when they are close together, 250⁄1000 of an inch, and that the bar be divided into inches and fiftieths. Now set the zero of the vernier opposite to the line denoting 49⁄50 inch on the bar. What, then, is the measurement between the outside edges of the jaws? In this case we require to add the 250⁄1000 to the 49⁄50 in order to read the measurement in terms of fiftieths and thousandths of an inch, or we may read the measurement to one hundredths of an inch, thus: 49⁄50 equal 98⁄100, and 250⁄1000 equal 25⁄100, and 98⁄1000 added to 25⁄100 are 123⁄100, or an inch and 23⁄100. To read in 1⁄1000ths of an inch, we have that 49⁄50 of an inch are equal to 980⁄1000, because each 1⁄50 inch contains 20⁄1000 inch, and this added to 250⁄1000 makes 1230⁄1000, that is 1230⁄1000 inches.
The accuracy of the instrument may be maintained, notwithstanding any wear which may in the course of time take place on the inside faces of the jaws, by adjusting the zero line on the vernier to exactly coincide with the zero line on the bar, but the fineness of the lines renders this a difficult matter with the naked eye, hence it is desirable to read the instrument with the aid of a magnifying glass. If the outer edges of the jaws should wear, it is simply necessary to alter the allowance made for their widths.
Fig. 1394.
[Fig. 1394] represents standard plug and collar gauges. These tools are made to represent exact standard measurements, and obviously do no more than to disclose whether the piece measured is exactly to size or not. If the work is not to size they will not determine how much the error or difference is, hence they are gauges rather than measuring tools. It is obvious, however, that if the work is sufficiently near to size, the plug or male gauge may be forced in, or the collar or female gauge may be forced on, and in this case the tightness of the fit would indicate that the work was very near to standard size. But the use of such gauges in this way would rapidly wear them out, causing the plug gauge and also the collar to get smaller than its designated size, hence such gauges are intended to fit the work without friction, and at the same time without any play or looseness whatever. Probably the most accurate degree of fit would be indicated when the plug gauge would fit into the collar sufficiently to just hold its own weight when brought to rest while within the collar, and then slowly fall through if put in motion within the collar. It is obvious that both the plug and the collar cannot theoretically be of the same size or one would not pass within the other, but the difference that is sufficient to enable this to be done is so minute that it is practically too small to measure and of no importance.
Fig. 1395.
When these gauges are used by the workmen, to fit the work to their wear is sufficient to render it necessary to have some other standard gauge to which they can be from time to time referred to test their accuracy, and for this purpose a standard such as in [Fig. 1395] may be employed. It consists of a number of steel disks mounted on an arbor and carefully ground after hardening each to its standard size.