Basic Working Principle of Vernier Calipers

We have thoroughly established that the Vernier Caliper is an amazingly versatile tool which enables the measurement of lengths with a much greater degree of accuracy than other tools. The Vernier caliper provides this function along with the ability to measure depth, and inside and outside radius measurements of objects. Not to mention it is available in various sizes ranging from 150 mm to as long as 2 metres. Considering all this it is easy to see why the tool is still in popular use despite its origination way back in 1631 by French scientist Pierre Vernier.

The Vernier Caliper is constructed of a few simple components that fit together to make it work. These parts are not very complex and have been perfected over the years to give performance at low costs. However as is the case with most tools, the better and smoother the performance gets, the higher the price gets. The Vernier Calipers main components are its jaws.

There are two types of jaws, the upper and lower jaws which are used to measure inside and outside dimensions respectively. One of these jaws is moving while the other is fixed. The fixed jaw is connected to the main scale of the caliper while the moving jaw is connected to the Vernier scale.

The Main body or frame of the Vernier caliper contains a large scale that runs down its length. This scale is known as the main scale and is graduated in centimetres. The smallest value or the least count of the main scale is 1 millimetre. The vernier scale is smaller than the main scale and also contains up to 50 graduations. The vernier scale graduations are used to further divide the least measurement of the main scale i.e if 50 divisions are present the value of 1mm can be further divided into 50 parts.

The Vernier scale uses the basic principle of alignment of line segments to improve the accuracy of the readings. When a certain graduation on the vernier scale aligns with a graduation on the main scale. The value of the vernier scale reading is added to the main scale reading to obtain the decimal value of the reading in millimetres.

This is made possible by the difference in the spacing between the graduations on both the scale. The main scale is graduated like a normal meter rule with each mark at a distance of 1 mm. However the vernier scale has different distance between the graduations. This spacing is normally of 0.9 mm. In this way if the zero markings on the vernier scale is aligned with the zero on the main scale, then the first mark on the vernier scale will be 0.1 mm before the first mark on the main scale.

Similarly the second mark will be 0.1*2  = 0.2 mm short of the corresponding mark on the main scale. This will carry on for the subsequent marks with the third mark 0.3 mm before, the fourth 0.4mm till the 10th mark on the vernier scale which will be exactly 1 mm behind the 10th mark on the main scale.

Let us now imagine that we are measuring an object 8.7 mm in length. The jaws will be opened that length and the vernier scale will slide forward on the main scale. The zero mark of the Vernier scale will slide a total of 8.7 mm and will end up somewhere between the 8 and 9 mm marks on the vernier scale. In fact it will be ahead of the 8mm mark by 0.7 mm.

The next mark on the vernier scale which was previously behind the corresponding mark on the main scale by 0.1 mm will now be 0.7-0.1= 0.6 mm ahead. Similarly the 2nd mark will be 0.7-0.2= 0.5 mm ahead. However the 7th mark on the vernier scale which was behind its corresponding mark by 0.7 mm will now move ahead and become perfectly aligned with that mark on the main scale. Thus the value of the vernier scale will be 0.7mm and this will be added to the main scale reading of 8 to give the reading of 8.7 mm.

This principle works because the alignment can be detected easily by the human eye and noted to find the accurate measurement. So you can rest easy now that you have understood the mystery behind the simple genius behind the working of the Vernier Caliper.