Protractor on the dial from 180°, as the dial is graduated from opposite zero marks to 90° each way. Since the spaces, both on the main scale and the Vernier scale, are numbered both to the right and to the left from zero, any angle can be measured. The readings can be taken either to the right or to the left, according to the direction in which the zero on the main scale is moved. Reading the Vernier scale: The bevel protractor Vernier scale may have graduations of 5′ (minutes) or 1/12°. Each space on the Vernier scale is 5′ less
Leiden scale Leiden scale The Leiden scale (°L) was used to calibrate low-temperature indirect measurements in the early twentieth century, by providing conventional values (in kelvins, then termed "degrees Kelvin") of helium vapour pressure. It was used below -183°C, the starting point of the International Temperature scale in the 1930s (Awbery 1934). The Leiden scale probably goes back to around 1894, when Heike Kamerlingh Onnes' cryogenic laboratory was established in Leiden, Netherlands. It has been reported that the scale is the kelvin scale shifted so that the boiling points of hydrogen and oxygen become zero and 70 respectively, but this is unlikely