Mainspring gauge

They were made by several European companies and are all similar to the gauge in Figure 1, being brass plates with notches on both edges and round sinks on both sides.

The strength is determined by finding the smallest round sink into which the mainspring barrel (horology) will fit.

About 1840 Aaron Lufkin Dennison devised a gauge "upon which all the different parts of a watch could be accurately measured", Figure 3.

The edges have the usual notches for mainspring heights (based on the Imperial inch, but there is no obvious way to determine mainspring strength, except perhaps by barrel diameter using one of the three scales on the top left (which are Imperial inch scales in 1/64 and 1/32 divisions).

The slit gauge is also based on the Imperial inch, but it often has metric equivalents which are crude approximations.

Figure 1: Martin mainspring gauge (front and back views). Actual size 181 x 30 mm.
Figure 2: Martin mainspring slit gauge. Actual size 87 x 17 mm.
Figure 3: Dennison combined gauge. Actual size 150 × 19 mm.
Figure 4: Dennison simplified mainspring gauge