The many kinds of spherometers differ in the device used (sometimes mechanical, sometimes optical and sometimes electric) to establish the moment of contact.
It must be kept in mind that when the spherometer is not equipped with any particular device and an artifice for determining the moment of contact cannot be found, if the screw is lowered too much the instrument acquires great freedom of movement and easily rotates around the point of the screw; it then suffices to turn the screw in the other direction and, proceeding division by division, first in one direction and then in the other, to determine the instant of contact with an uncertainty of no more than a few divisions.