Gauges having simple notches are not suitable for measuring accurately the thickness of metal, because the edges of the sheets or plates frequently vary from the thickness of the body of the plate. This may occur from the wear of the rolls employed to roll out the sheet, or because the sheets have been sheared to cut them to the required width, or to remove cracks at the edges, which shearing is apt to form a burr or projection on one side of the edge, and a slight depression on the other.

Again, a gauge formed by a notch requires to slide over the metal of the plate, and friction and a wear causing an enlargement of the notch ensues, which destroys the accuracy of the gauge. To avoid this source of error the form of gauge that was shown in [Fig. 1370] may be used, it having the further advantage that it will measure thicknesses intermediate between the sizes of two contiguous notches, thus measuring the actual thickness of the sheet when it is not to any accurate sheet metal gauge thickness.

It is to be observed that in the process of rolling, the sheet is reduced from a greater to a lesser thickness, hence the gauge will not pass upon the plate until the latter is reduced to its proper thickness.

In applying the gauge, therefore, there is great inducement for the workman to force the gauge on to the sheet, in order to ascertain how nearly the sheet is to the required size, and this forcing process causes rapid wear to the gauge.

It follows, therefore, that a gauge should in no case be forced on, but should be applied lightly and easily to the sheet to prevent wear. Here may be mentioned another advantage of the Brown and Sharpe gauge, in that its gauge-number measurements being uniform, it may be more readily known to what extent a given plate varies from its required gauge thickness.

Suppose, for example, a sheet requiring to be of Number 1 Birmingham gauge is above the required thickness, but will pass easily through the 0 notch of the gauge, the excessive variation of those two gauge numbers (over the variations between other consecutive numbers of the gauge) leaves a wider margin in estimating how much the thickness is excessive than would be the case in using the Brown and Sharpe gauge. Indeed, if the edge of the plate be of uniform thickness with the body of the plate, the variation from the required thickness may be readily ascertained by a Brown and Sharpe gauge, by the distance the plate will pass up the aperture beyond the line denoting the 0 gauge number, or by the distance it stands from the 1 on the gauge when passed up the aperture until it meets both sides of the same.

In addition to these standard gauges, some firms in the United States employ a standard of their own; the principal of these are given in comparison with others in the table following.

DIMENSIONS OF SIZES, IN DECIMAL PARTS OF AN INCH.

Number
of
Wire
Gauge.		American
or Brown
& Sharpe.		Birming-
ham, or
Stubs’s.		Washburn
& Moen
Mfg. Co.,
Worcester,
Ms.		Trenton
Iron Co.,
Trenton,
N. J.		G. W.
Prentiss,
Holyoke,
Mass.		Old English,
from Brass
Manu-
facturers’
List.
000000————.46——————
00000————.43.45————
0000.46.454.393.4————
000.40964.425.362.36.3586——
00.3648.38.331.33.3282——
0.32495.34.307.305.2994——
1.2893.3.283.285.2777——
2.25763.284.263.265.2591——
3.22942.259.244.245.2401——
4.20431.238.225.225.223——
5.18194.22.207.205.2047——
6.16202.203.192.19.1885——
7.14428.18.177.175.1758——
8.12849.165.162.16.1605——
9.11443.148.148.145.1471——
10.10189.134.135.13.1351——
11.090742.12.12.1175.1205——
12.080808.109.105.105.1065——
13.071961.095.092.0925.0928——
14.064084.083.08.08.0816.083
15.057068.072.072.07.0726.072
16.05082.065.063.061.0627.065
17.045257.058.054.0525.0546.058
18.040303.049.047.045.0478.049
19.03539.042.041.039.0411.04
20.031961.035.035.034.0351.035
21.028462.032.032.03.0321.0315
22.025347.028.028.027.029.0295
23.022571.025.025.024.0261.027
24.0201.022.023.0215.0231.025
2S.0179.02.02.019.0212.023
26.01594.018.018.018.0194.0205
27.014195.016.017.017.0182.01875
28.012641.014.016.016.017.0165
29.011257.013.015.015.0163.0155
30.010025.012.014.014.0156.01375
31.008928.01.0135.013.0146.01225
32.00795.009.013.012.0136.01125
33.00708.008.011.011.013.01025
34.006304.007.01.01.0118.0095
35.005614.005.0095.009.0109.009
36.005.004.009.008.01.0075
37.004453——.0085.00725.0095.0065
38.003965——.008.0065.009.00575

In the Whitworth wire-gauge, the mark or number on the gauge simply denotes the number of 1⁄1000ths of an inch the wire is in diameter; thus Number 1 on the gauge is 1⁄1000 inch, Number 2 is 2⁄1000ths inch in diameter, and so on.